Identification of bitter ligands that specifically activate human T2R receptors and related assays for identifying human bitter taste modulators

ABSTRACT

The present invention relates to the discovery that specific human taste receptors in the T2R taste receptor family respond to particular bitter compounds. Also, the invention relates to the discovery of specific hT2R9 alleles and their disparate activity in functional assays with the same bitter ligands. The invention further relates to the use of these T2R receptors in assays for identifying ligands that modulate the activation of these taste receptors by specific bitter ligands and related compounds. These compounds may be used as additives and/or removed from foods, beverages, cosmetics and medicinals in order to modify (block) T2R-associated bitter taste. Also T2R ligands may be used as therapeutics to treat and modulate T2R associated gastrointestinal and metabolic functions as well as treat gastrointestinal and metabolic diseases such as eating disorders, food sensing, food absorption, obesity, diabetes, Crohn&#39;s disease, celiac disease, et al.

RELATED APPLICATIONS

This application claims priority to and is a divisional of U.S. Ser. No.14/243,380, which is a divisional of U.S. Ser. No. 13/019,580 filed Feb.2, 2011, now U.S. Pat. No. 8,175,946, which is a continuation of U.S.Ser. No. 11/766,974 filed Jun. 22, 2007, now U.S. Pat. No. 7,883,856,which is a continuation-in-part of U.S. Ser. No. 11/555,617 filed onNov. 1, 2006, now U.S. Pat. No. 8,030,008, which is in turn is acontinuation-in-part of U.S. Ser. No. 10/191,058 filed Jul. 10, 2002,now U.S. Pat. No. 7,338,771 all of which applications are incorporatedby reference in their entireties herein. These applications relate tothe identification of hT2Rs and the use thereof in assays for theidentification of ligands that activate specific T2Rs. These ligands areuseful for modulating taste perception, particularly bitter taste.

SEQUENCE LISTING

This application includes as part of its disclosure a biologicalsequence listing text file which is being submitted via EFS-Web. Saidbiological sequence listing is contained in the file named“43268o2612.txt” having a size of 100,919 bytes that was created Aug.26, 2015, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the elucidation of bitter compoundsthat activate a number of previously reported human G-protein coupledreceptors (GPCRs) in the T2R family that are involved in bitter tasteperception. Specifically, the invention involves the discovery of bitterligands that specifically bind and activate hT2R1, hT2R3, hT2R4, hT2R5,hT2R7, hT2R8, hT2R9, hT2R10, hT2R13, hT2R14, hT2R16, hT2R44, hT2R50,hT2R51, hT2R54, hT2R55, hT2R61, hT2R64, hT2R65, hT2R67, hT2R71, hT2R75,and hT2R76. Therefore, the above-identified human T2Rs may be used toidentify compounds that modulate, preferably block, the bitter tasteassociated with these and other ligands.

More specifically, the present discoveries indicate that the subjecthuman taste receptors, fragments, or variants or chimeras thereof,including orthologs, splice variants, single nucleotide polymorphisms(SNPS), and genetically engineered mutants thereof, are useful inassays, preferably high throughput cell-based assays, for identifyingcompounds that modulate (preferably block) the bitter taste of bitterligands, as well as structurally related compounds and other compoundsthat activate these receptors. Compounds identified using these assaysmay be used as additives in foods, beverages or medicinal products toimprove the taste thereof. Additionally, the invention relates tomodified foods, beverages and medicinals that are treated and formulatedin order to reduce or eliminate bitter compounds that activate thesubject T2Rs.

Also, the present invention relates to the use of the subject T2R genesand the corresponding polypeptides and cells which express same intherapeutic screens, e.g., for identifying compunds that can be used tomodulate gastrointestinal functions such as food sensing, absorption,regulation of gastrointestinal hormone and peptide secretion, transportand absorption, responses to toxins in the lingual and gastrointestinalsystems, treatment of gastrointestinal and metabolic disorders such aseating disorders, diabetes, obesity, and the like.

DESCRIPTION OF THE RELATED ART

One of the basic taste modalities that humans can recognize is bitter.The physiology of bitter taste until quite recently was very poorlyunderstood. Recent studies have started to shed light on the biology oftaste (Lindemann, Nature (2001)). It is now known that many bittercompounds produce bitter taste by interacting with cell surfacereceptors. These receptors belong to the family of seven transmembranedomain receptors that interact with intracellular G proteins. A novelfamily of GPCRs, termed T2Rs, has been identified in humans and rodents(Adler et al., Cell 100(6):693-702 (2000); Chandrashekar et al., Cell100(6): 703-711 (2000); Matsunami H, Montmayeur J P, Buck L B. Nature404(6778): 601-4 (2000)). Several lines of evidence prior to the subjectinvention suggested that T2Rs mediate responses to bitter compounds.First, T2R genes are specifically expressed in subset of taste receptorcells of the tongue and palate epithelia. Second, the gene for one ofthe human T2Rs (hT2R1) is located in a chromosomal locus that is linkedto sensitivity to bitter compound 6-n-propyl-2-thiouracil in humans(Adler et al., (Id.) (2000)). Third, one of the mouse T2Rs (mT2R5) islocated in a chromosomal locus that is linked to sensitivity to bittercompound cycloheximide in mice. It was also shown that mT2R5 canactivate gustducin, G protein specifically expressed in taste cells andlinked to bitter stimuli transduction (Wong et al., Nature 381:796-800(1996)). Gustducin activation by mT2R5 occurs only in response tocycloheximide (Chandrashekar et al., (Id.) (2000). Thus, it has beenproposed that mT2R family mediates bitter taste response in mice,whereas hT2R family mediates bitter taste response in humans. Only onehuman T2R was suggested as having identified bitter ligand-hT2R4 wasshown as being activated by denatonium (Chandrashekar et al., (Id.)2000). However, effective denatonium concentrations used in the study(1.5 mM) were unusually high, i.e., is 10⁵-fold higher than the reportedbitter threshold for denatonium to humans (Saroli, Naturwissenschaften71:428-429 (1984)). Thus, no specific bitter ligand was convincinglymatched to any hT2R. It has been also suggested that each hT2R is ableto bind multiple bitter ligands. This hypothesis is based on the factthat hT2R family consists of only 24 identified members, whereas humanscan recognize hundreds of different compounds as bitter. Sequences ofhT2Rs have been previously reported and are discloses in published PCTapplications by Zuker et al. (WO 01/18050 A2, (2001)) and Adler et al.(WO 01/77676 A1 (2001)) both of which are incorporated by reference intheir entirety herein.

One of the difficulties of studying T2R function is that these receptorsare not readily expressed in cultured mammalian cell lines. To improveT2R expression an N-terminal sequence from well-expressed GPCR,rhodopsin, was attached to T2R sequences (Chandrashekar et al., (Id.)2000). This N-terminal tag also allowed easy monitoring of proteinexpression due to available antibody. In addition, SSTR3 tag (Bufe etal., Nat. Genet. 32:397-400 (2002)), a different N-terminal tag has beenused to improve T2R expression. Whereas the incorporation of therhodopsin tag improved expression of some T2Rs in mammalian cell lines,many of them still were not expressed well enough for functionalstudies. In a different approach mT2R5 was successfully expressed ininsect Sf9 cells and used for functional studies using biochemical GTPγSbinding assay (Chandrashekar et al., (Id.) 2000).

In Applicants' earlier patent application, U.S. Ser. No. 09/825,882 nowpatented, Applicants identified and provided the nucleic acid sequencesand polypeptide sequences for a number of then-novel human tastereceptors including hT2R51, hT2R54, hT2R55, hT2R61, hT2R63, hT2R64,hT2R65, hT2R67, hT2R71, and hT2R75. Additionally in U.S. Ser.l No.10/628,464 now patented. Applicants provided the polypeptide and DNAsequence for another identified novel human taste receptor named thereinhT2R76.

Also, in U.S. Ser. No. 10/191,058 incorporated by reference herein inits entirety, Applicants discovered ligands that specifically activatethree different human T2Rs. Additionally, Applicants recently filed U.S.Ser. No. 11/455,693 which further identified bitter ligands thatspecifically bind to other human T2Rs, and provided related assays.

Also, relating to practical utilities of the invention it has beenreported that both T2Rs and T1Rs taste receptors are ecxpressed in thegastrointestinal system For example, Wu et al., Proc. Natl. Acad. Sci.USA 99(4):2392-7(2002) report that T2Rs are expressed in enteroendocrinecells (STC1 cells) as well as gustducin and transducin subunits and thatthese cells likely respond to bitter ligands in the gastrointestinaltract. Also, it has been reported by Chen et al., AM J. Physiol. CellPhysol. 291(4):C726-39 (2006) that bitter taste stimuli induce Ca++signaling and cholecystokinin (CCK) release in enteroendocrine STC-1cells. Also, Rozengurt, Am J Physiol Gastrointes Liver Physiol291(2):G171-7 (2006) report that taste receptors in the gut likely playa role in molecular sending the control of digestive functions, andhormonal and/or neutronal pathways and that they may play a role in thedetection of harmful drugs and survival responses. Further, Sternini AmJ Physiol Gastrointest Liver Physiol. 292(2):G457-61 (2007) report thattaste receptors in the gut may be involved in gastrointestinal functionssuch as molecular sensing, nutrient absorption, protection from harmfulsubstances, and further suggest that an understanding of thesemechanisms may be relevant to disease states and coinditions such asfeeding disorders, and inflammation. Further, it has been recentlysuggested by Mace et al, J Physiol. 2007 [Epub] that T2Rs and T1Rsactivate phospholipase C beta 2, PLC beta2, and that there is likely amolecular intestinal sensing system in the gut similar to that presentin lingual cells and that gastrointestinal cells such as brush cells orsolitary chemosensory cells expressing taste receptors may result inGLUT2 increase and may play a role in nutrient sensing, and nutrition inthe treatment of obesity and diabetes. Also, Cui et al, Curr Pharm Des.12(35):4591-600 (2006) suggesst that T1Rs expressed in the gut may beused in assays for compounds in treating obesity and diabetes as well asartificial sweeteners.

However, notwithstanding what has been reported and the understandingthat T2R members regulate bitter taste, and their possible role ingastrointestinal functions there exists a need for the identification ofspecific ligands which activate human bitter T2R taste receptors. Agreater understanding of the binding properties of different T2Rs,particularly human T2Rs, would be highly beneficial as it will greaterfacilitate the use thereof in selecting compounds having desired tastemodulatory properties, i.e., which block or inhibit the taste ofspecific bitter compounds. Also, it will provide for the identificationof compounds for treating and modulating gastrointestinal functions andrelated diseases such as obesity, diabetes, food absorption, foodsensing, eating disorders, and in the regulation of related hormones andpeptides such as GLUT2, cholecystokin et al.

SUMMARY OF THE INVENTION

Toward that end, the present invention relates to the discovery ofligands that specifically bind and/or activate a total of 23 human tastereceptors in the T2R family, particularly hT2R1, hT2R3, hT2R4, hT2R5,hT2R7, hT2R8, hT2R9, hT2R10, hT2R13, hT2R14, hT2R16, hT2R44, hT2R50,hT2R51, hT2R54, hT2R55, hT2R61, hT2R64, hT2R65, hT2R67, hT2R71, hT2R75and hT2R76.

These discoveries were made using cell-based assays that measured theactivity of T2Rs using cells that express a particular T2R in thepresence and absence of specific bitter ligands. In particular, asdescribed in greater detail infra, HEK cell lines expressing theabove-identified specific T2Rs on their surface and which furtherexpressed a chimeric G protein that functionally couple to said T2Rswere used in cell-based assays that detected changes in intracellularcalcium concentrations, and were found to be specifically activated byspecific bitter compounds whereas other hT2Rs were not activated undersimilar conditions.

Therefore, the invention embraces the use of these human taste receptorsin assays, preferably high-throughput assays, to identify compounds thatmodulate, preferably block, the activation of these receptors by theseand other bitter compounds.

Also, the invention relates to the use of these receptors to identifycompounds that elicit a bitter taste.

Further, the present invention relates to the use of the subject T2Rsand corresponding polypeptides and cells which express same intherapeutic screening assays, e.g., for identifying compounds thatregulate or modulate gastrointestinal functions such as food andnutrient sensing, food absorption, regulation of digestive hormones andpeptides, responses to toxins, and for treating gastrointestinal ormetabolic diseases such as obesity, diabetes, and inflamatory orautoimmune gastrointestinal dieases such as IBD, celiac disease,Crohn'sdisease, et al.

The invention also embraces assays which include an additional stepwhich evaluates the effect of the identified modulating compounds inhuman or other taste tests, and evaluates the effect of the identifiedcompounds on bitter taste tests and/or in further in vitro or in vivoclinical tests to evaluate the effect of an identified compound on aspecific gastrointestinal, digestive or metabolic function or disease.Also, the invention embraces the use of the identified compounds infoods, beverages and medicines as flavor or taste modulators, i.e., toinhibit bitter taste, e.g., the bitter taste associated with specificbeverages and foods or medicaments. Further, the invention embraces theproduction of food, beverages and medicinals which have been treated toremove compounds that specifically activate bitter taste receptors,e.g., foods and beverages that have been processed to remove or reducethe amount of bitter compounds comprised therein. Still further thepresent invention embraces medicaments containing identified compoundssuitable for treating or preventing metabolic disorders, digestivefunctions, and gastrointestinal diseases involving T2Rs. Particularly,the present invention contemplates medicaments for treating ormodulating conditions such as Crohn's, celiac disease, obesity,diabetes, food sensing, food absorption, digestive hormone or peptidesecretion, and the like.

U.S. Ser. No. 10/742,209 filed on Dec. 1, 2003 and its parentapplication U.S. Ser. No. 09/825,882 filed on Apr. 5, 2001, now U.S.Pat. No. 7,105,650, are each hereby incorporated by reference in theirentireties.

OBJECTS OF THE INVENTION

It is an object of the invention to provide assays that identifycompounds which activate or which block or modulate the activation of atleast one hT2R selected from hT2R1, hT2R3, hT2R4, hT2R5, hT2R7, hT2R8,hT2R9, hT2R10, hT2R13, hT2R14, hT2R16, hT2R44, hT2R50, hT2R51, hT2R54,hT2R55, hT2R61, hT2R64, hT2R65, hT2R67, hT2R71, hT2R75 and hT2R76 orfragments, variants, orthologs, or chimeras thereof by bitter ligands,including the specific bitter ligands disclosed herein found tospecifically bind and activate these human bitter taste receptors.

It is a specific object of the invention to provide assays that identifycompounds which activate or which block or modulate the activationand/or binding of hT2R1 or fragments, variants, or chimeras thereof byat least one of chloramphenicol, chloroquine, cyclooctanone,dexamethasone, diltiazem, hydrochloride, Ginkgolide A, Lomefloxacin,N-methylthiourea, nitrosaccharin, methylprednisone, oleuropein,meprazole, oxybutynin chloride, oxyphenomium HBr, peptide-LPFNQL (SEQ IDNO: 51), peptide-LPFSQL (SEQ ID NO: 52), Peptide-YQEPVLGPVRGVRGPFPIIV(SEQ ID NO: 53), peptide PVLGPVRGFPIIV (SEQ ID NO: 54), peptidePVRGPFPIIV (SEQ ID NO: 55), peptide RGPFPIIV (SEQ ID NO: 56), picricacid, prednisone, quinine, sulfamethoxazole, thioacetanilide,thiocarbanilide and other structurally related or bitter compounds.

It is another specific object of the invention to provide assays thatidentify compounds which activate or which block or modulate theactivation and/or binding of hT2R3 or a fragment, ortholog, variant orchimera thereof by at least one of 2′acetylpyrazine, chloroquine orlomefloxacin or other structurally related and bitter compounds.

It is another object of the invention to provide assays that identifycompounds which activate or which block or modulate the activationand/or binding of hT2R4 by at least one compound selected from4-benzylpiperidine, chloroquine, diltiazem hydrochloride,diisobutylamine, 2,6-dimethylpiperidine, doxepin, labetalol HCl,(−)lupinine, 1-methyl-2-quinolinone, methylprednisolone, oleuropein,omeprazole, oxybutynin chloride, oxyphenonium HBr, pirenzepinedihydrochloride, procainamide, quinine, ranitidine, strychnine,theobromine, tolazoline, trimethoprim, and L-tryptophan and otherstructurally related or bitter compounds.

It is another object of the invention to provide assays that identifycompounds which activate or which block or modulate the activationand/or binding to hT2R5 by at least one compound selected fromdimethylbiguanide, 1-methyl-2-quinolinone, oleuropein, and 2-picolineand other structurally related and bitter compounds.

It is another object of the invention to provide assays that identifycompounds which activate or which block or modulate the activationand/or binding of hT2R7 with at least one compound selected from2-acetylpyrazine, chloroquine, ethylpyrazine, 1-methy-2-quinolinone,oxybutynin chloride, oxyphenonium HBr, 2-picoline, pirenzepinedihydrochloride, quinine, strychnine, trimethoprim and otherstructurally related or bitter compounds.

It is another object of the invention to provide assays that identifycompound which activate or which block or modulate the activation and/orbinding of hT2R8 with at least one compound selected from acesulfame K,2-acetylpyrazine, aloin, andrographolide, atropine, chloramphenicol,cycloheximide, cyclooctanone, denatonium benzoate, dexamethasone,diltiazem hydrochloride, enalapril maleate, (−)erythromycin,ethylpyrazine, famotidine, gabapentin, ginkgolide A, goitrin, guaiacolglyceryl ether, lomefloxacin, 1-meth-2-quinolinone, methylprednisolone,nitrophthalene, nitrosaccharin, oleuropein, oxybutynin chloride,oxyphenonium HBr, N′-ethyl-N′-phenylurea, picric acid, pirenzepinedihydrochloride, prednisone, quinacrine, ranitidine, saccharin, sucroseoctaacetate, strychnine, tolylurea, and trimethoprim and otherstructurally related or bitter compounds.

It is another object of the invention to provide assays that identifycompounds which activate or which block or modulate the activationand/or binding of hT2R9 by at least one compound selected fromethylpyrazine, ofloxacin, and ranitidine and other structurally relatedor bitter compounds.

It is another specific object of the invention to provide assays thatidentify compounds which activate or which block or modulate theactivation and/or binding of hT2R10 to at least one compound selectedfrom 2-acetylpyrazine, andrographolide, atropine, brucine,4-benzylpiperidine, caffeine, chloramphenicol, chloroquine, cinchonine,clarithromycin, clindamycin, cycloheximide, cyclooctanone, den atoniumbenzoate, dexamethasone, diltiazem hydrochloride, diisobutylamine,2,6-dimethylpiperidine, doxepin, edrophonium, (−)erythromycin,ethylpyrazine, famotidine, gabapentin, ginkgolide A. goitrin, guaicolglyceryl ether, (−)lupinine, 1-methy-2-quinolinone, methylprednisolone,oleuropein, omeprazole, oxybutynin chloride, oxypherionium HBr,procainamide, prednisone, quassin, quinacrine, quinine, ranitidine,spartein sulfate pentahydrate, sucrose octaacetate, strychnine,tolazoline, tolylurea, trapidil, and trimethoprim, chloride and otherstructurally related or bitter compounds.

It is another object of the invention to provide assays that identifycompounds which activate or which block or modulate the activationand/or binding of hT2R13 to at least one compound selected from2-acetylpyrazine, atropine, clarithromycin, denatonium benzoate,doxepin, ethylpyrazine, oleuropein, oxyphenonium HBr, and quinacrine andother structurally related or bitter compounds.

It is another object of the invention to provide assays that identifycompounds that activate or which block or modulate the activation and/orbinding of hT2R14 to at least one compound selected from2-acetylpyrazine, arisolochic acid, cyclooctanone, dexamethasone,diltiazem hydrochloride, 2,6-dimethylpiperidine, erythromycin,ethylpyrazine, goitrin, guaicol glyceryl ether, 1-methy-2-quinolinone,methylprednisolone, nitronaphthalene, nitrosaccharin, oleuropein,omeprazole, oxybutynin chloride, N′-ethyl-N′-phenylurea, 2-picoline,picric acid, quinine, strychnine, theobromine, tolylurea, and trapidiland other structurally related or bitter compounds.

It is another object of the invention to provide assays which identifycompounds that activate or which block or modulate the activation and/orbinding of hT2R16 to at least one compound selected from2-acetylpyrazine, amygadalin, arbutin, linamarin, and D-(−)-salicin andother structurally related or bitter compounds.

It is another object of the invention to provide assays which identifycompounds that activate or which block or modulate the activation and/orbinding of hT2R44 to at least one compound selected from2-acetylpyrazine or ethylpyrazine and other structurally related orbitter compounds.

It is another object of the invention to provide assays which identitycompounds that activate or which block or modulate the activation and/orbinding of hT2R50 to at least one compound selected from2-acetylpyrazine or ethylpyrazine or other structurally related orbitter compounds.

It is another object of the invention to provide assays which identifycompounds that activate or which block or modulate the activation and/orbinding of hT2R54 to at least one compound selected from acetaminophen,chloroquine, clarithromycin, denatonium benzoate. (−)-epicatechin,(−)-erythromycin, labetalol-HCl, oleuropein, omeprazole, oxybutyninchloride, oxyphenonium HBr, pirenzepine dihydrochloride, procainamide,ranitidine, strychnine, trimethoprim, and L-tryptophan and otherstructurally related or bitter compounds.

It is another object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R55 to at least one compound selected from doxepin,linamarin, oxybutynin chloride, quinine, strychnine, and trimethoprimand other structurally related or bitter compounds.

It is another object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R61 by at least one compound selected from acesulfame K,aloin, aristolochic acid, caffeine, chloramphenicol, chloroquine,denatonium, benzoate, nitrooxybutinin chloride, oxyphenonium,peptide-LPFNQL (SEQ ID NO: 51), peptide-LPFSQL (SEQ ID NO: 52), Picricacid, saccharin, and strychnine and other structurally related or bittercompounds.

It is also an object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R64 by at least one compound selected from acesulfame k,amino-2-norbornane-carboxylic acid, aristolochic acid,2,6-dimethylpiperidine, quinine, ranitidine, saccharin, strychnine, andL-tryptophan and other structurally related or bitter compounds.

It is also an object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R65 by at least one compound selected from2-acetylpyrazine, ethylpyrazine, and 1-methy-2-quinolinone and otherstructurally related or bitter compounds.

It is also an object of the invention to provide assays for identifyingcompounds that block or modulate the activation and/or binding of hT2R67by at least one compound selected from 2-acetylpyrazine,andrographolide, ethylpyrazine, and oxybutynin chloride and otherstructurally related or bitter compounds.

It is also an object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R71 by at least one compound selected from nitrosaccharin,and picric acid and other structurally related or bitter compounds.

It is another object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R75 by at least one compound selected fromandrographolide, atropine, brucine, 4-benzylpiperidine, caffeine,chloramphenicol, chloroquine, cinchonine, ciprofloxacin denatoniumbenzoate, dexamethasone, doxepin, enalapril maleate, enoxacin,prednisone, procainamide, quassin, quinine, ranitidine, spartein sulfatepentahydrate, strychnine, sulfamethoxazole, trapidil, and trimethoprimand other structurally related or bitter compounds.

It is another object of the invention to provide assays for identifyingcompounds that activate or which block or modulate the activation and/orbinding of hT2R76 by at least one compound selected from brucine , andother structurally related or bitter compounds

It is a specific object of the invention to use cells or cell membranesthat comprise or express (stably or transiently) at least one of hT2R1,hT2R3, hT2R4, hT2R5, hT2R7, hT2R8, hT2R9, hT2R10, hT2R13, hT2R14,hT2R16, hT2R44, hT2R50, hT2R51, hT2R54, hT2R55, hT2R61, hT2R64, hT2R65,hT2R67, hT2R71, hT2R75 and hT2R76 or a fragment, variant, ortholog,mutant or chimera thereof in assays to identify compounds that activateor which block or modulate the activation of at least one of saidreceptor by one of the above-identified bitter compounds or anotherstructurally related or bitter compound.

It is an even more specific object of the invention to use cells,preferably mammalian, amphibian or insect cells, e.g., HEK293T cellsthat express a G protein that couples thereto, e.g., G_(α15), G_(α16),gustducin or a chimera thereof, e.g., G_(α16) gustducin or transducinchimeric G protein in cell-based assays that detect changes inintracellular calcium order to detect compounds that activate or whichmodulate, preferably block or inhibit, the activation of one of theafore-mentioned human taste receptors by one of the afore-mentionedbitter compounds or another structurally related or bitter compound.

It is another object of the invention to confirm that the identifiedcompounds modulate, preferably inhibit or block, bitter taste, e.g. thatelicited by the afore-identified bitter compounds or other structurallyrelated bitter compounds in human or animal taste tests, preferablyhuman taste tests.

It is another object of the invention to utilize compounds identified inthe assays described herein as additives or flavor modulators incompositions in order to inhibit or block the bitter taste elicited bycompounds that specifically activate these taste receptors. A preferredobject of the invention is to use a compound that inhibits activation ofat least one of the above-identified human T2R receptors in order toblock the bitter taste of compounds present in some foods, beverages,cosmetics and medicinals.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 contains a Table which compares the nomenclature used bydifferent groups in referring to human bitter taste receptors (hT2Rs).

FIGS. 2A-C contains a Table which tabulates the results of calciumimaging experiments which identified specific bitter ligands that bindand specifically activate the hT2Rs identified therein.

FIGS. 3A-B contains a table summarizing the particular bitter ligandstested as well as the concentrations used in the calcium imaging assaysthe results of which are summarized in FIG. 2.

FIG. 4 compares the functional activity of 2 different hT2R9 alleles(hT2R9A ansd hT2R9V) in HEK-293 cells using the bitter ligand ofloxacin.

FIG. 5 compares the functional activity of the same 2 hT2R9 allelesusing a different expression system comprising the SSTR label andtransiently transfected cells and detection of hT2R9 activity usingFLIPR and a different bitter ligand (ranitidine).

DETAILED DESCRIPTION OF THE INVENTION

Prior to specifically describing the invention, the followingdefinitions are provided.

The term “T2R” family includes polymorphic variants, alleles, mutants,and homologs that: (1) have about 30-40% amino acid sequence identity,more specifically about 40, 50, 60, 70, 75, 80, 85, 90, 95, 96, 97, 98,or 99% amino acid sequence identity to the T2Rs disclosed infra, and inthe Zuker (Id) (2001) and Adler (Id.) (2001) applications incorporated,by reference herein over a window of about 25 amino acids, optimally50-100 amino acids; (2) specifically hind to antibodies raised againstan immunogen comprising an amino acid sequence selected from the groupconsisting of the T2R sequences disclosed infra, and conservativelymodified variants thereof; (3) specifically hybridize (with a size of atleast about 100, optionally at least about 500-1000 nucleotides) understringent hybridization conditions to a sequence selected from the groupconsisting of the T2R DNA sequences disclosed infra, and conservativelymodified variants thereof; (4) comprise a sequence at least about 40%identical to an amino acid sequence selected from the group consistingof the T2R amino acid sequences disclosed infra or (5) are amplified byprimers that specifically hybridize under stringent hybridizationconditions to the described T2R sequences.

In particular, these “T2R's” include taste receptor GPCRs referred toherein as hT2R1, hT2R3, hT2R4, hT2R5, hT2R7, hT2R8, hT2R9A, hT2R9V,hT2R10, hT2R13, hT2R14, hT2R16, hT2R44, hT2R50, hT2R54, hT2R55, hT2R61,hT2R64, hT2R65, hT2R67, hT2R71, hT2R75, and hT2R76 having the nucleicacid sequences and amino acid sequences provided in this application,and variants, alleles, mutants, orthologs and chimeras thereof whichspecifically bind to bitter ligands which are identified herein andother structurally related compounds and bitter compounds.

As noted in the Table in FIG. 1 the hT2Rs herein have also been referredto in the literature by other names. Herein when Applicants refer to aT2R sequence the Senomyx nomenclature is intended.

While T2R genes exhibit substantial sequence divergence at both theprotein and DNA level, all T2Rs isolated to date have been found tocontain certain consensus sequences in particular regions that areidentical or which possess or at least 70-75% sequence identity to theT2R consensus sequence identified previously in the Adler et al (WO01/77676 A1 (2001) and Zuker et al. WO 01/18050 A2, both incorporated byreference in their entirety herein.

Topologically, certain chemosensory GPCRs have an “N-terminal domain;”“extracellular domains,” a “transmembrane domain” comprising seventransmembrane regions, and corresponding cytoplasmic and extracellularloops, “cytoplasmic regions,” and a “C-terminal region” (see, e.g., Hoonet al, Cell, 96:541-51 (1999); Buck & Axel, Cell, 65:175-87 (1991)).These regions can be structurally identified using methods known tothose of skill in the art, such as sequence analysis programs thatidentify hydrophobic and hydrophilic domains (see, e.g., Stryer,Biochemistry, (3rd ed. 1988); see also any of a number of Internet basedsequence analysis programs, such as those found atdot.imgen.bcm.tmc.edu). These regions are useful for making chimericproteins and for in vitro assays of the invention, e.g., ligand bindingassays. For example chimeric T2Rs can be made by combining theextracellular region of one T2R and the transmembrane region of anotherT2R of the same or different species.

“Extracellular domains” therefore refers to the domains of T2Rpolypeptides that protrude from the cellular membrane and are exposed tothe extracellular face of the cell. Such regions would include the“N-terminal domain” that is exposed to the extracellular face of thecell, as well as the extracellular loops of the transmembrane domainthat are exposed to the extracellular face of the cell, i.e., theextracellular loops between transmembrane regions 2 and 3, transmembraneregions 4 and 5, and transmembrane regions 6 and 7. The “N-terminaldomain” starts at the N-terminus and extends to a region close to thestart of the transmembrane region. These extracellular regions areuseful for in vitro ligand binding assays, both soluble and solid phase.In addition, transmembrane regions, described below, can also beinvolved in ligand binding, either in combination with the extracellularregion or alone, and are therefore also useful for in vitro ligandbinding assays.

“T2R Expresing Cell” herein encompasses recombinant cells which exprss ahuman T2R sequence according to the invention as well as endogenous T2Rexpressing cells. Such cells are comprised in the lingual andgastrointestinal system and include cells in the oral cavity such astaste buds expressed on the tongue as well as cells in thegastrointestinal system and asociated organs such as brush cells in thegastrointestinal tract, enteroendocrine cells such as STC-1 cells. Thesecells may also express a G protein such as gustducin, transducin,Galpha15 or Galpha16. Cells which express specific T2Rs can beidentified and isolated by known methods such as by FACS cell separationof magnetic bead cell isolation procedures.

“Transmembrane domain,” which comprises the seven transmembrane“regions,” refers to the domain of T2R polypeptides that lies within theplasma membrane, and may also include the corresponding cytoplasmic(intracellular) and extracellular loops, also referred to astransmembrane “regions.” The seven transmembrane regions andextracellular and cytoplasmic loops can be identified using standardmethods, as described in Kyte & Doolittle, J. Mol. Biol., 157:105-32(1982)), or in Stryer, supra.

“Cytoplasmic domains” refers to the domains of T2R proteins that facethe inside of the cell, e.g., the “C-terminal domain” and theintracellular loops of the transmembrane domain, e.g., the intracellularloops between transmembrane regions 1 and 2, transmembrane regions 3 and4, and transmembrane regions 5 and 6. “C-terminal domain” refers to theregion that spans from the end of the last transmembrane region to theC-terminus of the protein, and which is normally located within thecytoplasm.

The term “7-transmembrane receptor” means a polypeptide belonging to asuperfamily of transmembrane proteins that have seven regions that spanthe plasma membrane seven times (thus, the seven regions are called“transmembrane” or “TM” domains TM I to TM VII). The families ofolfactory and certain taste receptors each belong to this super-family.7-transmembrane receptor polypeptides have similar and characteristicprimary, secondary and tertiary structures, as discussed in furtherdetail below.

The term “ligand-binding region” refers to sequences derived from achemosensory or taste receptor that substantially incorporatestransmembrane domains II to VII (TM II to VII). The region may becapable of binding a ligand, and more particularly, a taste elicitingcompound.

The term “plasma membrane translocation domain” or simply “translocationdomain” means a polypeptide domain which when incorporated into theamino terminus of a polypeptide coding sequence, can with greatefficiency “chaperone” or “translocate” the hybrid (“fusion”) protein tothe cell plasma membrane. For example a particular “translocationdomain” initially derived from the amino terminus of the human rhodopsinreceptor polypeptide, a 7-transmembrane receptor can be used. Anothertranslocation domain has been derived from the bovine rhodopsin sequenceand is also useful for facilitating translocation. Rhodopsin derivedsequences are particularly efficient in translocating 7-transmembranefusion proteins to the plasma membrane.

“Functional equivalency” means the domain's ability and efficiency intranslocating newly translated proteins to the plasma membrane asefficiently as an exemplary translocation domain such as one derivedfrom rhodopsin under similar conditions; relative efficiencies can bemeasured (in quantitative terms) and compared, as described herein.Domains falling within the scope of the invention can be determined byroutine screening for their efficiency in translocating newlysynthesized polypeptides to the plasma membrane in a cell (mammalian,Xenopus, and the like) with the same efficiency as the twenty amino acidlong translocation domain SEQ ID NO:1.

The phrase “functional effects” in the context of assays for testingcompounds that modulate T2R family member mediated taste transductionincludes the determination of any parameter that is indirectly ordirectly under the influence of the receptor, e.g., functional, physicaland chemical effects. It includes ligand binding, changes in ion flux,membrane potential, current flow, transcription, G protein binding, GPCRphosphorylation or dephosphorylation, signal transduction,receptor-ligand interactions, second messenger concentrations (e.g.,cAMP, cGMP, IP3, or intracellular Ca²⁺), in vitro, in vivo, and ex vivoand also includes other physiologic effects such increases or decreasesof neurotransmitter or hormone release.

By “determining the functional effect” is meant assays for a compoundthat increases or decreases a parameter that is indirectly or directlyunder the influence of a T2R family member, e.g., functional, physicaland chemical effects. Such functional effects can be measured by anymeans known to those skilled in the art, e.g., changes in spectroscopiccharacteristics (e.g., fluorescence, absorbance, refractive index),hydrodynamic (e.g., shape), chromatographic, or solubility properties,patch clamping, voltage-sensitive dyes, whole cell currents,radioisotope efflux, inducible markers, oocyte T2R gene expression;tissue culture cell T2R expression; transcriptional activation of T2Rgenes; ligand binding assays; voltage, membrane potential andconductance changes; ion flux assays; changes in intracellular secondmessengers such as cAMP, cGMP, and inositol triphosphate (IP3); changesin intracellular calcium levels; neurotransmitter release, and the like.

“Inhibitors,” “activators,” and “modulators” of T2R proteins receptorsare used interchangeably to refer to inhibitory, activating, ormodulating molecules identified using in vitro and in vivo assays fortaste transduction, e.g., ligands, agonists, antagonists, and theirhomologs and mimetics. Inhibitors are compounds that, e.g., bind to,partially or totally block stimulation, decrease, prevent, delayactivation, inactivate, desensitize, or down regulate tastetransduction, e.g., antagonists. Activators are compounds that, e.g.,bind to, stimulate, increase, open, activate, facilitate, enhanceactivation, sensitize, or up regulate taste transduction, e.g.,agonists. Modulators include compounds that, e.g., alter the interactionof a receptor with extracellular proteins that bind activators orinhibitor (e.g., ebnerin and other members of the hydrophobic carrierfamily); G Proteins; kinases (e.g., homologs of rhodopsin kinase andbeta adrenergic receptor kinases that are involved in deactivation anddesensitization of a receptor); and arrestins, which also deactivate anddesensitize receptors. Modulators include genetically modified versionsof T2R family members, e.g., with altered activity, as well as naturallyoccurring and synthetic ligands, antagonists, agonists, small chemicalmolecules and the like.

Such assays for inhibitors and activators include, e.g., expressing T2Rfamily members in cells or cell membranes, applying putative modulatorcompounds in the presence or absence of compounds that modulate, e.g.,bitter compounds, and then determining the functional effects on tastetransduction, as described above. Samples or assays comprising T2Rfamily members that are treated with a potential activator, inhibitor,or modulator are compared to control samples without the inhibitor,activator, or modulator to examine the extent of modulation. Controlsamples (untreated with modulators) are assigned a relative T2R activityvalue of 100%. Inhibition of a T2R is achieved when the T2R activityvalue relative to the control is about 80%, optionally 50% or 25-0%.Activation of a T2R is achieved when the T2R activity value relative tothe control is 110%, optionally 150%, optionally 200-500%, or 1000-3000%higher.

The terms “purified,” “substantially purified,” and “isolated” as usedherein refer to the state of being free of other, dissimilar compoundswith which the compound of the invention is normally associated in itsnatural state. Preferably, “purified,” “substantially purified,” and“isolated” means that the composition comprises at least 0.5%, 1%, 5%,10%, or 20%, and most preferably at least 50% or 75% of the mass, byweight, of a given sample. In one preferred embodiment, these termsrefer to the compound of the invention comprising at least 95% of themass, by weight, of a given sample. As used herein, the terms“purified,” “substantially purified,” and “isolated”, when referring toa nucleic acid or protein, of nucleic acids or proteins, also refers toa state of purification or concentration different. than that whichoccurs naturally in the mammalian, especially human, body. Any degree ofpurification or concentration greater than that which occurs naturallyin the mammalian, especially human, body, including (1) the purificationfrom other associated structures or compounds or (2) the associationwith structures or compounds to which it is not normally associated inthe mammalian, especially human, body, are within the meaning of“isolated.” The nucleic acid or protein or classes of nucleic acids orproteins, described herein, may be isolated, or otherwise associatedwith structures or compounds to which they are not normally associatedin nature, according to a variety of methods and processes known tothose of skill in the art.

As used herein, the term “isolated,” when referring to a nucleic acid orpolypeptide refers to a state of purification or concentration differentthan that which occurs naturally in the mammalian, especially human,body. Any degree of purification or concentration greater than thatwhich occurs naturally in the body, including (1) the purification fromother naturally-occurring associated structures or compounds, or (2) theassociation with structures or compounds to which it is not normallyassociated in the body are within the meaning of “isolated” as usedherein. The nucleic acids or polypeptides described herein may beisolated or otherwise associated with structures or compounds to whichthey are not normally associated in nature, according to a variety ofmethods and processed known to those of skill in the art.

As used herein, the terms “amplifying” and “amplification” refer to theuse of any suitable amplification methodology for generating ordetecting recombinant or naturally expressed nucleic acid, as describedin detail, below. For example, the invention provides methods andreagents (e.g., specific oligonucleotide primer pairs) for amplifying(e.g., by polymerase chain reaction, PCR) naturally expressed (e.g.,genomic or mRNA) or recombinant (e.g., cDNA) nucleic acids of theinvention (e.g., taste eliciting compound-binding sequences of theinvention) in vivo or in vitro.

The term “expression vector” refers to any recombinant expression systemfor the purpose of expressing a nucleic acid sequence of the inventionin vitro or in vivo, constitutively or inducibly, in any cell, includingprokaryotic, yeast, fungal, plant, insect or mammalian cell. The termincludes linear or circular expression systems. The term includesexpression systems that remain episomal or integrate into the host cellgenome. The expression systems can have the ability to self-replicate ornot, i.e., drive only transient expression in a cell. The term includesrecombinant expression “cassettes which contain only the minimumelements needed for transcription of the recombinant nucleic acid.

The term “library” means a preparation that is a mixture of differentnucleic acid or poly-peptide molecules, such as the library ofrecombinant generated sensory, particularly taste receptorligand-binding regions generated by amplification of nucleic acid withdegenerate primer pairs, or an isolated collection of vectors thatincorporate the amplified ligand-binding regions, or a mixture of cellseach randomly transfected with at least one vector encoding an tastereceptor.

The term “nucleic acid” or “nucleic acid sequence” refers to adeoxy-ribonucleotide or ribonucleotide oligonucleotide in either single-or double-stranded form. The term encompasses nucleic acids, i.e.,oligonucleotides, containing known analogs of natural nucleotides. Theterm also encompasses nucleic-acid-like structures with syntheticbackbones.

Unless otherwise indicated, a particular nucleic acid sequence alsoimplicitly encompasses conservatively modified variants thereof (e.g.,degenerate codon substitutions) and complementary sequences, as well asthe sequence explicitly indicated. Specifically, degenerate codonsubstitutions may be achieved by generating, e.g., sequences in whichthe third position of one or more selected codons is substituted withmixed-base and/or deoxyinosine residues (Batzer et al., Nucleic AcidRes., 19:5081 (1991); Ohtsuka et al., J. Biol. Chem., 260:2605-08(1985); Rossolini et al., Mol. Cell. Probes, 8:91-98 (1994)). The termnucleic acid is used interchangeably with gene, cDNA, mRNA,oligonucleotide, and polynucleotide.

The terms “polypeptide,” “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical mimetic of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers and non-naturally occurring amino acid polymer.

The “translocation domain,” “ligand-binding region”, and chimericreceptors compositions described herein also include “analogs,” or“conservative variants” and “mimetics” (“peptidomimetics”) withstructures and activity that substantially correspond to the exemplarysequences. Thus, the terms “conservative variant” or “analog” or“mimetic” refer to a polypeptide which has a modified amino acidsequence, such that the change(s) do not substantially alter thepolypeptide's (the conservative variant's) structure and/or activity, asdefined herein. These include conservatively modified variations of anamino acid sequence, i.e., amino acid substitutions, additions ordeletions of those residues that are not critical for protein activity,or substitution of amino acids with residues having similar properties(e.g., acidic, basic, positively or negatively charged, polar ornon-polar, etc.) such that the substitutions of even critical aminoacids does not substantially alter structure and/or activity.

More particularly, “conservatively modified variants” applies to bothamino acid and nucleic acid sequences. With respect to particularnucleic acid sequences, conservatively modified variants refers to thosenucleic acids which encode identical or essentially identical amino acidsequences, or where the nucleic acid does not encode an amino acidsequence, to essentially identical sequences. Because of the degeneracyof the genetic code, a large number of functionally identical nucleicacids encode any given protein.

For instance, the codons GCA, GCC, GCG and GCU all encode the amino acidalanine. Thus, at every position where an alanine is specified by acodon, the codon can be altered to any of the corresponding codonsdescribed without altering the encoded polypeptide.

Such nucleic acid variations are “silent variations,” which are onespecies of conservatively modified variations. Every nucleic acidsequence herein which encodes a polypeptide also describes everypossible silent variation of the nucleic acid. One of skill willrecognize that each codon in a nucleic acid (except AUG, which isordinarily the only codon for methionine, and TGG, which is ordinarilythe only codon for tryptophan) can be modified to yield a functionallyidentical molecule. Accordingly, each silent variation of a nucleic acidwhich encodes a polypeptide is implicit in each described sequence.

Conservative substitution tables providing functionally similar aminoacids are well known in the art. For example, one exemplary guideline toselect conservative substitutions includes (original residue followed byexemplary substitution): ala/gly or ser; arg/lys; asnlgln or his;asp/glu; cys/ser; gin/asn; gly/asp; gly/ala or pro; his/asn or gln;ile/leu or val; leu/ile or val; lys/arg or gln or glu; met/leu or tyr orile; phe/met or leu or tyr; ser/thr; thr/ser; trp/tyr; tyr/trp or phe;val/ile or leu. An alternative exemplary guideline uses the followingsix groups, each containing amino acids that are conservativesubstitutions for one another: 1) Alanine (A), Serine (S), Threonine(T); 2) Aspartic acid (D), Glutamic acid (E); 3) Asparagine (N),Glutamine (Q); 4) Arginine (R), Lysine (I); 5) Isoleucine (I), Leucine(L), Methionine (M), Valine (V); and 6) Phenylalanine (F), Tyrosine (Y),Tryptophan (W); (see also, e.g., Creighton, Proteins, W. H. Freeman andCompany (1984); Schultz and Schimer, Principles of Protein Structure,Springer-Verlag (1979)). One of skill in the art will appreciate thatthe above-identified substitutions are not the only possibleconservative substitutions. For example, for some purposes, one mayregard all charged amino acids as conservative substitutions for eachother whether they are positive or negative. In addition, individualsubstitutions, deletions or additions that alter, add or delete a singleamino acid or a small percentage of amino acids in an encoded sequencecan also be considered “conservatively modified variations.”

The terms “mimetic” and “peptidomimetic” refer to a synthetic chemicalcompound that has substantially the same structural and/or functionalcharacteristics of the polypeptides, e.g., translocation domains,ligand-binding regions, or chimeric receptors of the invention. Themimetic can be either entirely composed of synthetic, non-naturalanalogs of amino acids, or may be a chimeric molecule of partly naturalpeptide amino acids and partly non-natural analogs of amino acids. Themimetic can also incorporate any amount of natural amino acidconservative substitutions as long as such substitutions also do notsubstantially alter the mimetic's structure and/or activity.

As with polypeptides of the invention which are conservative variants,routine experimentation will determine whether a mimetic is within thescope of the invention, i.e., that its structure and/or function is notsubstantially altered. Polypeptide mimetic compositions can contain anycombination of non-natural structural components, which are typicallyfrom three structural groups: a) residue linkage groups other than thenatural amide bond (“peptide bond”) linkages; b) non-natural residues inplace of naturally occurring amino acid residues; or c) residues whichinduce secondary structural mimicry, i.e., to induce or stabilize asecondary structure, e.g., a beta turn, gamma turn, beta sheet, alphahelix conformation, and the like. A polypeptide can be characterized asa mimetic when all or some of its residues are joined by chemical meansother than natural peptide bonds. Individual peptidomimetic residues canbe joined by peptide bonds, other chemical bonds or coupling means, suchas, e.g., glutaraldehyde, N-hydroxysuccinimide esters, bifunctionalmaleimides, N,N′-dicyclohexylcarbodiimide (DCC) orN,N′-diisopropylcarbodiimide (DIC). Linking groups that can be analternative to the traditional amide bond (“peptide bond”) linkagesinclude, e.g., ketomethylene (e.g., —C(.═O)—CH₂ for —C(.═O)—NH—),aminomethylene (CH₂NH), ethylene, olefin (CH.dbd.CH), ether (CH₂O),thioether (CH₂—S), tetrazole (CN₄), thiazole, retroamide, thioamide, orester (see, e.g., Spatola, Chemistry and Biochemistry of Amino Acids,Peptides and Proteins, Vol. 7, 267-357, Marcell Dekker, Peptide BackboneModifications, NY (1983)). A polypeptide can also be characterized as amimetic by containing all or some non-natural residues in place ofnaturally occurring amino acid residues; non-natural residues are welldescribed in the scientific and patent literature.

A “label” or a “detectable moiety” is a composition detectable byspectroscopic, photochemical, biochemical, immunochemical, or chemicalmeans. For example, useful labels include ³²P, fluorescent dyes,electron-dense reagents, enzymes (e.g., as commonly used in an ELISA),biotin digoxigenin, or haptens and proteins which can be madedetectable, e.g., by incorporating a radiolabel into the peptide or usedto detect antibodies specifically reactive with the peptide.

A “labeled nucleic acid probe or oligonucleotide” is one that is bound,either covalently, through a linker or a chemical bond, ornoncovalently, through ionic, van der Waals, electrostatic, or hydrogenbonds to a label such that the presence of the probe may be detected bydetecting the presence of the label bound to the probe.

As used herein a “nucleic acid probe or oligonucleotide” is defined as anucleic acid capable of binding to a target nucleic acid ofcomplementary sequence through one or more types of chemical bonds,usually through complementary base pairing, usually through hydrogenbond formation. As used herein; a probe may include natural (i.e., A, G,C, or T) or modified bases (7-deazaguanosine, inosine, etc.). Inaddition, the bases in a probe may be joined by a linkage other than aphosphodiester bond, so long as it does not interfere withhybridization. Thus, for example, probes may be peptide nucleic acids inwhich the constituent bases are joined by peptide bonds rather thanphosphodiester linkages. It will be understood by one of skill in theart that probes may bind target sequences lacking completecomplementarity with the probe sequence depending upon the stringency ofthe hybridization conditions. The probes are optionally directly labeledas with isotopes, chromophores, lumiphores, chromogens, or indirectlylabeled such as with biotin to which a streptavidin complex may laterbind. By assaying for the presence or absence of the probe, one candetect the presence or absence of the select sequence or subsequence,

The term “heterologous” when used with reference to portions of anucleic acid indicates that the nucleic acid comprises two or moresubsequences that are not found in the same relationship to each otherin nature. For instance, the nucleic acid is typically recombinantlyproduced, having two or more sequences from unrelated genes arranged tomake a new functional nucleic acid, e.g., a promoter from one source anda coding region from another source. Similarly, a heterologous proteinindicates that the protein comprises two or more subsequences that arenot found in the same relationship to each other in nature (e.g., afusion protein).

A “promoter” is defined as an array of nucleic acid sequences thatdirect transcription of a nucleic acid. As used herein, a promoterincludes necessary nucleic acid sequences near the start site oftranscription, such as, in the case of a polymerase II type promoter, aTATA element. A promoter also optionally includes distal enhancer orrepressor elements, which can be located as much as several thousandbase pairs from the start site of transcription. A “constitutive”promoter is a promoter that is active under most environmental anddevelopmental conditions. An “inducible” promoter is a promoter that isactive under environmental or developmental regulation. The term“operably linked” refers to a functional linkage between a nucleic acidexpression control sequence (such as a promoter, or array oftranscription factor binding sites) and a second nucleic acid sequence,wherein the expression control sequence directs transcription of thenucleic acid corresponding to the second sequence.

As used herein, “recombinant” refers to a polynucleotide synthesized orotherwise manipulated in vitro (e.g., “recombinant polynucleotide”), tomethods of using recombinant polynucleotides to produce gene products incells or other biological systems, or to a polypeptide (“recombinantprotein”) encoded by a recombinant polynucleotide. “Recombinant means”also encompass the ligation of nucleic acids having various codingregions or domains or promoter sequences from different sources into anexpression cassette or vector for expression of, e.g., inducible orconstitutive expression of a fusion protein comprising a translocationdomain of the invention and a nucleic acid sequence amplified using aprimer of the invention.

The phrase “selectively (or specifically) hybridizes to” refers to thebinding, duplexing, or hybridizing of a molecule only to a particularnucleotide sequence under stringent hybridization conditions when thatsequence is present in a complex mixture (e.g., total cellular orlibrary DNA or RNA).

The phrase “stringent hybridization conditions” refers to conditionsunder which a probe will hybridize to its target subsequence, typicallyin a complex mixture of nucleic acid, but to no other sequences.Stringent conditions are sequence dependent and will be different indifferent circumstances. Longer sequences hybridize specifically athigher temperatures. An extensive guide to the hybridization of nucleicacids is found in Tijssen, Techniques in Biochemistry and MolecularBiology—Hybridization with Nucleic Probes, “Overview of principles ofhybridization and the strategy of nucleic acid assays” (1993).Generally, stringent conditions are selected to be about 5-10° C. lowerthan the thermal melting point (Tm) for the specific sequence at adefined ionic strength pH. The Tm is the temperature (under definedionic strength, pH, and nucleic concentration) at which 50% of theprobes complementary to the target hybridize to the target sequence atequilibrium (as the target sequences are present in excess, at Tm, 50%of the probes are occupied at equilibrium). Stringent conditions will bethose in which the salt concentration is less than about 1.0 M sodiumion, typically about 0.01 to 1.0 M sodium ion concentration (or othersalts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. forshort probes (e.g., 10 to 50 nucleotides) and at least about 60° C. forlong probes (e.g., greater than 50 nucleotides). Stringent conditionsmay also be achieved with the addition of destabilizing agents such asformamide. For selective or specific hybridization, a positive signal isat least two times background, optionally 10 times backgroundhybridization. Exemplary stringent hybridization conditions can be asfollowing: 50% formamide, 5×SSC, and 1% SDS, incubating at 42° C. or,5×SSC, 1% SDS, incubating at 65° C., with wash in 0.2×SSC, and 0.1% SDSat 65° C. Such hybridizations and wash steps can be carried out for,e.g., 1, 2, 5, 10, 15, 30, 60; or more minutes.

Nucleic acids that do not hybridize to each other under stringentconditions are still substantially related if the polypeptides whichthey encode are substantially related. This occurs, for example, when acopy of a nucleic acid is created using the maximum codon degeneracypermitted by the genetic code. In such cases, the nucleic acidstypically hybridize under moderately stringent hybridization conditions.Exemplary “moderately stringent hybridization conditions” include ahybridization in a buffer of 40% formamide, 1 M NaCl, 1% SDS at 37° C.,and a wash in 1×SSC at 45° C. Such hybridizations and wash steps can becarried out for, e.g., 1, 2, 5, 10, 15, 30, 60, or more minutes. Apositive hybridization is at least twice background. Those of ordinaryskill will readily recognize that alternative hybridization and washconditions can be utilized to provide conditions of similar stringency.

“Antibody” refers to a polypeptide comprising a framework region from animmunoglobulin gene or fragments thereof that specifically binds andrecognizes an antigen. The recognized immunoglobulin genes include thekappa, lambda, alpha, gamma, delta, epsilon, and mu constant regiongenes, as well as the myriad immunoglobulin variable region genes. Lightchains are classified as either kappa or lambda. Heavy chains areclassified as gamma, mu, alpha, delta, or epsilon, which in turn definethe immunoglobulin classes, IgG, IgM, IgA, IgD and IgE, respectively.

An exemplary immunoglobulin (antibody) structural unit comprises atetramer. Each tetramer is composed of two identical pairs ofpolypeptide chains, each pair having one “light” (about 25 kDa) and one“heavy” chain (about 50-70 kDa). The N-terminus of each chain defines avariable region of about 100 to 110 or more amino acids primarilyresponsible for antigen recognition. The terms variable light chain(V_(L)) and variable heavy chain (V_(H)) refer to these light and heavychains respectively.

A “chimeric antibody” is an antibody molecule in which (a) the constantregion, or a portion thereof, is altered, replaced or exchanged so thatthe antigen binding site (variable region) is linked to a constantregion of a different or altered class, effector function and/orspecies, or an entirely different molecule which confers new propertiesto the chimeric antibody, e.g., an enzyme, toxin, hormone, growthfactor, drug, etc.; or (b) the variable region, or a portion thereof, isaltered, replaced or exchanged with a variable region having a differentor altered antigen specificity.

An “anti-T2R” antibody is an antibody or antibody fragment thatspecifically binds a polypeptide encoded by a T2R gene, cDNA, or asubsequence thereof.

The term “immunoassay” is an assay that uses an antibody to specificallybind an antigen. The immunoassay is characterized by the use of specificbinding properties of a particular antibody to isolate, target, and/orquantify the antigen.

The phrase “specifically (or selectively) binds” to an antibody or,“specifically (or selectively) immunoreactive with,” when referring to aprotein or peptide, refers to a binding reaction that is determinativeof the presence of the protein in a heterogeneous population of proteinsand other biologics. Thus, under designated immunoassay conditions, thespecified antibodies bind to a particular protein at least two times thebackground and do not substantially bind in a significant amount toother proteins present in the sample. Specific binding to an antibodyunder such conditions may require an antibody that is selected for itsspecificity for a particular protein.

For example, polyclonal antibodies raised to a T2R family member fromspecific species such as rat, mouse, or human can be selected to obtainonly those polyclonal antibodies that are specifically immunoreactivewith the T2R polypeptide or an immunogenic portion thereof and not withother proteins, except for orthologs or polymorphic variants and allelesof the T2R polypeptide. This selection may be achieved by subtractingout antibodies that cross-react with T2R molecules from other species orother T2R molecules. Antibodies can also be selected that recognize onlyT2R GPCR family members but not GPCRs from other families. A variety ofimmunoassay formats may be used to select antibodies specificallyimmunoreactive with a particular protein. For example, solid-phase ELISAimmunoassays are routinely used to select antibodies specificallyimmunoreactive with a protein (see, e.g., Harlow & Lane, Antibodies, ALaboratory Manual, (1988), for a description of immunoassay formats andconditions that can be used to determine specific immunoreactivity).Typically a specific or selective reaction will be at least twicebackground signal or noise and more typically more than 10 to 100 timesbackground.

The phrase “selectively associates with” refers to the ability of anucleic acid to “selectively hybridize” with another as defined above,or the ability of an antibody to “selectively (or specifically) bind toa protein, as defined above.

The term “expression vector” refers to any recombinant expression systemfor the purpose of expressing a nucleic acid sequence of the inventionin vitro or in vivo, constitutively or inducibly, in any cell, includingprokaryotic, yeast, fungal, plant, insect or mammalian cell. The termincludes linear or circular expression systems. The term includesexpression systems that remain episomal or integrate into the host cellgenome. The expression systems can have the ability to self-replicate ornot, i.e., drive only transient expression in a cell. The term includesrecombinant expression “cassettes which contain only the minimumelements needed for transcription of the recombinant nucleic acid.

By “host cell” is meant a cell that contains an expression vector andsupports the replication or expression of the expression vector. Hostcells may be prokaryotic cells such as E. coli, or eukaryotic cells suchas yeast, insect, amphibian, or mammalian cells such as CHO, HeLa,HEK-293, and the like, e.g., cultured cells, explants, and cells invivo.

Based on the foregoing, the present invention provides assays foridentifying compounds that modulate, preferably block, the specificactivation of the previously identified human bitter taste receptor bybitter compounds, e.g., the bitter compounds identified in FIGS. 2 and 3and structurally related and other bitter compounds. Particularly, theinvention provides cell-based assays for identifying compounds thatmodulate (e.g., block) the activation of hT2R1, hT2R3, hT2R4, hT2R5,hT2R7, hT2R8, hT2R9, hT2R10, hT2R13, hT2R14, hT2R14, hT2R16, hT2R44,hT2R50, hT2R51, hT2R54, hT2R55, hT2R61, hT2R64, hT2R65, hT2R67, hT2R71,hT2R75, hT2R76 by one of the bitter ligands shown to activate thesespecific taste receptors identified in FIG. 2 or another structurallyrelated or another bitter compound. These compounds will modulate bittertaste associated with these taste receptors in human subjects. This willbe confirmed in taste tests.

That the above taste receptors specifically respond to at least one ofthe bitter ligands contained in FIG. 2 was determined essentially usingthe HEK293 expression system and calcium imaging methods reported inother publications as well as patent applications filed by the presentAssignee, e.g., U.S. Ser. Nos. 10/191,058 and 09/825,882, bothincorporated by reference in their entireties herein. More particularly,the present inventors transfected HEK293 cells with a particular hT2Rtagged with a rhodopsin 35 amino acid tag (SEQ ID NO:1) together with achimeric G protein (G16gust44) which comprises the G_(α15) G proteinsequence modified by the replacement of carboxy-44 amino acid residueswith those of gustducin, and recorded responses of these cells tospecific bitter ligands by calcium imaging methods.

Specifically, the inventors used a mammalian cell-based assay to monitorhT2R activities. For calcium imaging assays, cells were seeded into48-well tissue culture plates. 24 hours later the cells were transientlytransfected with an expression plasmid (pEAK10) containing an hT2Rnucleic acid sequence, and a plasmid (pEAK10) containing a chimeric Gprotein (G16gust44). Another 24 hours later the cells were incubatedwith a fluorescent dye specific for calcium (Fluo-4; Molecular Probes).The loaded cells are exposed o different bitter molecules, and theactivation of an hT2R leads to activation of G16gust44, which in turnleads to calcium mobilization inside within the cells. This increase incalcium concentration changes the fluorescence properties of the calciumdye inside the cells. These changes are monitored using fluorescencemicroscopy.

The inventors also used the automated fluorimetric aiming system FLIPRusing a slightly different protocol. A HEK293 cell line stablyexpressing G16gust44 was transfected with a hT2R expression plasmid, 24hours later, the cells are loaded and analyzed on FLIPR.

After a ligand is identified for a particular hT2R, a HEK293 cell linestably expressing both the hT2R and G16gust44 are generated facilitatingfuture screening assays to identify other ligands that activate theparticular hT2R or which modulate (block or enhance) the activation ofthis hT2R by another bitter ligand such as a compound identified in FIG.2. This avoids the need for transient transfection.

As shown in the Figures such experiments identified ligand-receptorpairs for 23 different hT2Rs. It was found that most tested bittermolecules activated more than 1 hT2R, and also that most hT2Rs arebroadly “tuned” to respond to structurally diverse bitter molecules. Asnoted, all the identified bitter ligands and the concentrations at whichthey were tested in the calcium imaging assays is contained in FIG. 3.In addition, FIGS. 4 and 5 reveal that different T2R alleles may respondto bitter ligands differently and that such allelic variants may affectbitter taste and/or other T2R asociated activities in differentindividuals depending upon what T2R allele(s) they express.

These results indicate that cells which functionally express any one ofthe identified hT2R taste receptors may be used in assays to identifylegends that modulate bitter taste associated with at least one of saidparticular hT2Rs.

Preferably, these assays will utilize a test cell that expresses a DNAencoding an hT2R having one of the amino acid sequences identifiedinfra. However, it is anticipated that fragments, orthologs, variants orchimeras of these receptor polypeptides which retain the functionalproperties of these bitter taste receptors, i.e., respond to some bittercompounds, will also be useful in these assays. Examples of suchvariants include splice variants, single nucleotide polymorphisms,allelic variants, and mutations produced by recombinant or chemicalmeans, or naturally occurring. Means for isolation and expression ofT2Rs, which are used in the assays of the present invention and assayswhich are contemplated for use in the present invention to identifycompounds that inhibit activation of these receptors, are set forthbelow.

Isolation and Expression of T2Rs

Isolation and expression of the T2Rs, or fragments or variants thereof,of the invention can be effected by well-established cloning proceduresusing probes or primers constructed based on the T2R nucleic acidssequences disclosed in the application. Related T2R sequences may alsobe identified from human or other species genomic databases using thesequences disclosed herein and known computer-based search technologies,e.g., BLAST sequence searching. In a particular embodiment, thepseudogenes disclosed herein can be used to identify functional allelesor related genes.

Expression vectors can then be used to infect or transfect host cellsfor the functional expression of these sequences. These genes andvectors can be made and expressed in vitro or in vivo. One of skill willrecognize that desired phenotypes for altering and controlling nucleicacid expression can be obtained by modulating the expression or activityof the genes and nucleic acids (e.g., promoters, enhancers and the like)within the vectors of the invention. Any of the known methods describedfor increasing or decreasing expression or activity can be used. Theinvention can be practiced in conjunction with any method or protocolknown in the art, which are well described in the scientific and patentliterature.

Alternatively, these nucleic acids can be synthesized in vitro bywell-known chemical synthesis techniques, as described in, e.g.,Carruthers, Cold Spring Harbor Symp. Quant. Biol. 47:411-18 (1982);Adams, Am. Chem, Soc., 105:661 (1983); Belousov, Nucleic Acids Res.25:3440-3444 (1997); Frenkel, Free Radic. Biol. Med. 19:373-380 (1995);Blommers, Biochemistry 33:7886-7896 (1994); Narang, Meth. Enzymol. 68:90(1979); Brown, Meth. Enzymol. 68:109 (1979); Beaucage, Tetra. Lett.22:1859 (1981); U.S. Pat. No. 4,458,066. Double-stranded DNA fragmentsmay then be obtained either by synthesizing the complementary strand andannealing the strands together under appropriate conditions, or byadding the complementary strand using DNA polymerase with an appropriateprimer sequence.

Techniques for the manipulation of nucleic acids, such as, for example,for generating mutations in sequences, subcloning, labeling probes,sequencing, hybridization and the like are well described in thescientific and patent literature. See, e.g., Sambrook, ed., MolecularCloning: A Laboratory Manual (2nd ed.), Vols, 1-3, Cold Spring HarborLaboratory (1989); Ausubel, ed., Current Protocols in Molecular Biology,John Wiley & Sons, Inc., New York (1997); Tijssen, ed., LaboratoryTechniques in Biochemistry and Molecular Biology; Hybridization WithNucleic Acid Probes, Part I, Theory and Nucleic Acid Preparation,Elsevier, N.Y. (1993).

Nucleic acids, vectors, capsids, polypeptides, and the like can beanalyzed and quantified by any of a number of general means well knownto those of skill in the art. These include, e.g., analyticalbiochemical methods such as NMR, spectrophotometry, radiography,electrophoresis, capillary electrophoresis, high performance liquidchromatography (HPLC), thin layer chromatography (TLC), andhyperdiffusion chromatography, various immunological methods, e.g.,fluid or gel precipitin reactions, immunodiffusion,immunoelectrophoresis, radioimmunoassays (RIAs), enzyme-linkedimmunosorbent assays (ELISAs), immuno-fluorescent assays, Southernanalysis, Northern analysis, dot-blot analysis, gel electrophoresisSDS-PAGE), RT-PCR, quantitative PCR, other nucleic acid or target orsignal amplification methods, radiolabeling, scintillation counting, andaffinity chromatography.

Oligonucleotide primers may be used to amplify nucleic acids encoding aT2R ligand-binding region. The nucleic acids described herein can alsobe cloned or measured quantitatively using amplification techniques.Amplification methods are also well known in the art, and include, e.g.,polymerase chain reaction (PCR) (Innis ed., PCR Protocols, a Guide toMethods and Applications, Academic Press, N.Y. (1990); Innis ed., PCRStrategies, Academic Press, Inc., N.Y. (1995)); ligase chain reaction(LCR) (Wu, Genomics, 4:560 (1989); Landegren, Science, 241:1077 (1988);Barringer, Gene, 89:117 (1990)); transcription amplification (Kwoh,PNAS, 86:1173 (1989)); self-sustained sequence replication (Guatelli,PNAS, 87:1874 (1990)); Q Beta replicase amplification (Smith, J. Clin.Microbiol., 35:1477-91 (1997)); automated Q-beta replicase amplificationassay (Burg, Mol. Cell. Probes, 10:257-71 (1996)); and other RNApolymerase mediated techniques (e.g., NASBA, Cangene, Mississauga,Ontario). See also, Berger, Methods Enzymol., 152:307-16 (1987);Sambrook; Ausubel; U.S. Pat. Nos. 4,683,195 and 4,683,202; Sooknanan,Biotechnology, 13:563-64 (1995).

Once amplified, the nucleic acids, either individually or as libraries,may be cloned according to methods known in the art, if desired, intoany of a variety of vectors using routine molecular biological methods;methods for cloning in vitro amplified nucleic acids are described,e.g., U.S. Pat. No. 5,426,039. To facilitate cloning of amplifiedsequences, restriction enzyme sites can be “built into” the PCR primerpair. For example, Pst 1 and Bsp E1 sites were designed into theexemplary primer pairs of the invention. These particular restrictionsites have a sequence that, when ligated, are “in-frame” with respect tothe 7-membrane receptor “donor” coding sequence into which they arespliced (the ligand-binding region coding sequence is internal to the7-membrane polypeptide, thus, if it is desired that the construct betranslated downstream of a restriction enzyme splice site, out of frameresults should be avoided; this may not be necessary if the insertedligand-binding region comprises substantially most of the transmembraneVII region). The primers can be designed to retain the original sequenceof the “donor” 7-membrane receptor. Alternatively, the primers canencode amino acid residues that are conservative substitutions (e.g.,hydrophobic for hydrophobic residue, see above discussion) orfunctionally benign substitutions (e.g., do not prevent plasma membraneinsertion, cause cleavage by peptidase, cause abnormal folding ofreceptor, and the like).

The primer pairs may be designed to selectively amplify ligand-bindingregions of T2R. proteins. These binding regions may vary for differentligands; thus, what may be a minimal binding region for one ligand, maybe too limiting for a second potential ligand. Thus, binding regions ofdifferent sizes comprising different domain structures may be amplified;for example, transmembrane (TM) domains II through VII, III through VII,III through VI or II through VI, or variations thereof (e.g., only asubsequence of a particular domain, mixing the order of the domains, andthe like), of a 7-transmembrane T2R.

As domain structures and sequence of many 7-membrane T2R proteins areknown, the skilled artisan can readily select domain-flanking andinternal domain sequences as model sequences to design degenerateamplification primer pairs. For example, a nucleic acid sequenceencoding domain regions II through VII can be generated by PCRamplification using a primer pair. To amplify a nucleic acid comprisingtransmembrane domain I (TM I) sequence, a degenerate primer can bedesigned from a nucleic acid that encodes the amino acid sequence of theT2R family consensus sequence 1 described above. Such a degenerateprimer can be used to generate a binding region incorporating TM Ithrough TM TM I through TM IV, TM I through TM V, TM I through TM VI orTM I through TM VII). Other degenerate primers can be designed based onthe other T2R family consensus sequences provided herein. Such adegenerate primer can be used to generate a binding region incorporatingTM III through TM IV, TM III through TM V, TM III through TM VI or TMIII through TM VII.

Paradigms to design degenerate primer pairs are well known in the art.For example, a COnsensus-DEgenerate Hybrid Oligonucleotide Primer(CODEHOP) strategy computer program is accessible ashttp://blocks.there.org/codehop.html, and is directly linked from theBlockMaker multiple sequence alignment site for hybrid primer predictionbeginning with a set of related protein sequences, as known tastereceptor ligand-binding regions (see, e.g., Rose, Nucleic Acids Res.,26:1628-35 (1998); Singh, Biotechniques, 24:318-19 (1998)).

Means to synthesize oligonucleotide primer pairs are well known in theart. “Natural” base pairs or synthetic base pairs can be used. Forexample, use of artificial nucleobases offers a versatile approach tomanipulate primer sequence and generate a more complex mixture ofamplification products. Various families of artificial nucleobases arecapable of assuming multiple hydrogen bonding orientations throughinternal bond rotations to provide a means for degenerate molecularrecognition. Incorporation of these analogs into a single position of aPCR primer allows for generation of a complex library of amplificationproducts. See, e.g., Hoops, Nucleic Acids Res., 25:4866-71 (1997).Nonpolar molecules can also be used to mimic the shape of natural DNAbases. A non-hydrogen-bonding shape mimic for adenine can replicateefficiently and selectively against a nonpolar shape mimic for thymine(see, e.g., Morales, Nat. Struct. Biol., 5:950-54 (1998)). For example,two degenerate bases can be the pyrimidine base 6H,8H-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one or the purine baseN6-methoxy-2,6-diaminopurine (see, e.g., Hill, PNAS, 95:4258-63 (1998)).Exemplary degenerate primers of the invention incorporate the nucleobaseanalog5′-Dimethoxytrityl-N-benzoyl-2′-deoxy-Cytidine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite(the term “P” in the sequences, see above). This pyrimidine analoghydrogen bonds with purines, including A and G residues.

Polymorphic variants, alleles, and interspecies homologs that aresubstantially identical to a taste receptor disclosed herein can beisolated using the nucleic acid probes described above. Alternatively,expression libraries can be used to clone T2R polypeptides andpolymorphic variants, alleles, and interspecies homologs thereof, bydetecting expressed homologs immunologically with antisera or purifiedantibodies made against a T2R polypeptide, which also recognize andselectively bind to the T2R homolog.

Nucleic acids that encode ligand-binding regions of taste receptors maybe generated by amplification (e.g., PCR) of appropriate nucleic acidsequences using appropriate (perfect or degenerate) primer pairs. Theamplified nucleic acid can be genomic DNA from any cell or tissue ormRNA or cDNA derived from taste receptor-expressing cells.

In one embodiment, hybrid protein-coding sequences comprising nucleicacids encoding T2Rs fused to a translocation sequences may beconstructed. Also provided are hybrid T2Rs comprising the translocationmotifs and taste eliciting compound-binding regions of other families ofchemusensory receptors, particularly taste receptors. These nucleic acidsequences can be operably linked to transcriptional or translationalcontrol elements, e.g., transcription and translation initiationsequences, promoters and enhancers, transcription and translationterminators, polyadenylation sequences, and other sequences useful fortranscribing DNA into RNA. In construction of recombinant expressioncassettes, vectors, and transgenics, a promoter fragment can be employedto direct expression of the desired nucleic acid in all desired cells ortissues.

In another embodiment, fusion proteins may include C-terminal orN-terminal translocation sequences. Further, fusion proteins cancomprise additional elements, e.g., for protein detection, purification,or other applications. Detection and purification facilitating domainsinclude, e.g., metal chelating peptides such as polyhistidine tracts,histidine-tryptophan modules, or other domains that allow purificationon immobilized metals; maltose binding protein; protein A domains thatallow purification on immobilized immunoglobulin; or the domain utilizedin the FLAGS extension/affinity purification system (Immunex Corp,Seattle Wash.).

The inclusion of a cleavable linker sequences such as Factor Xa (see,e.g., Ottavi, Biochimie, 80:289-93 (1998)), subtilisin proteaserecognition motif (see, e.g., Polyak, Protein Eng., 10:615-19 (1997));enterokinase (Invitrogen, San Diego, Calif.), and the like, between thetranslocation domain (for efficient plasma membrane expression) and therest of the newly translated polypeptide may be useful to facilitatepurification. For example, one construct can include a polypeptideencoding a nucleic acid sequence linked to six histidine residuesfollowed by a thioredoxin, an enterokinase cleavage site (see, e.g.,Williams, Biochemistry, 34:1787-97 (1995)), and an C-terminaltranslocation domain. The histidine residues facilitate detection andpurification while the enterokinase cleavage site provides a means forpurifying the desired protein(s) from the remainder of the fusionprotein. Technology pertaining to vectors encoding fusion proteins andapplication of fusion proteins are well described in the scientific andpatent literature (see, e.g., Kroll, DNA Cell. Biol,. 12:441-53 (1993)).

Expression vectors, either as individual expression vectors or aslibraries of expression vectors, comprising the ligand-binding regionencoding sequences may be introduced into a genome or into the cytoplasmor a nucleus of a cell and expressed by a variety of conventionaltechniques, well described in the scientific and patent literature. See,e.g., Roberts, Nature, 328:731 (1987); Berger supra; Schneider, ProteinExper. Purif., 6435:10 (1995); Sambrook; Tijssen; Ausubel. Productinformation from manufacturers of biological reagents and experimentalequipment also provide information regarding known biological methods.The vectors can be isolated from natural sources, obtained from suchsources as ATCC or GenBank libraries, or prepared by synthetic orrecombinant methods.

The nucleic acids can be expressed in expression cassettes, vectors orviruses which are stably or transiently expressed in cells (e.g.,episomal expression systems). Selection markers can be incorporated intoexpression cassettes and vectors to confer a selectable phenotype ontransformed cells and sequences. For example, selection markers can codefor episomal maintenance and replication such that integration into thehost genome is not required. For example, the marker may encodeantibiotic resistance (e.g., chloramphenicol, kanamycin, G418,bleomycin, hygromycin) or herbicide resistance (e.g., chlorosulfurone orBasta) to permit selection of those cells transformed with the desiredDNA sequences (see, e.g., Blondelet-Rouault, Gene, 190:315-17 (1997);Aubrecht, J, Pharmacol. Exp. Ther., 281:992-97 (1997)). Becauseselectable marker genes conferring resistance to substrates likeneomycin or hygromycin can only be utilized in tissue culture,chemoresistance genes are also used as selectable markers in vitro andin vivo.

A chimeric nucleic acid sequence may encode a T2R ligand-binding regionwithin any 7-transmembrane polypeptide. Because 7-transmembrane receptorpolypeptides have similar primary sequences and secondary and tertiarystructures, structural domains (e.g., extracellular domain, TM domains,cytoplasmic domain, etc.) can be readily identified by sequenceanalysis. For example, homology modeling, Fourier analysis and helicalperiodicity detection can identify and characterize the seven domainswith a 7-transmembrane receptor sequence. Fast Fourier Transform (FFT)algorithms can be used to assess the dominant periods that characterizeprofiles of the hydrophobicity and variability of analyzed sequences.Periodicity detection enhancement and alpha helical periodicity indexcan be done as by, e.g., Donnelly, Protein Sci., 2:55-70 (1993). Otheralignment and modeling algorithms are well known in the art (see, e.g.,Peitsch, Receptors Channels, 4:161-64 (1996); Kyte & Doolittle, J. Md.Biol., 157:105-32 (1982); and Cronet, Protein Eng., 6:59-64 (1993).

The present invention also includes not only the nucleic acid moleculesand polypeptides having the specified nucleic and amino acid sequences,but also fragments thereof, particularly fragments of, e.g., 40, 60, 80,100, 150, 200, or 250 nucleotides, or more, as well as polypeptidefragments of, e.g., 10, 20, 30, 50, 70, 100, or 150 amino acids, ormore, Optionally, the nucleic acid fragments can encode an antigenicpolypeptide that is capable of binding to an antibody raised against aT2R family member. Further, a protein fragment of the invention canoptionally be an antigenic fragment that is capable of binding to anantibody raised against a T2R family member.

Also contemplated are chimeric proteins, comprising at least 10, 20, 30,50, 70, 100, or 150 amino acids, or more, of one of at least one of theT2R polypeptides described herein, coupled to additional amino acidsrepresenting all or part of another GPCR, preferably a member of the 7transmembrane superfamily. These chimeras can be made from the instantreceptors and another GPCR, or they can be made by combining two or moreof the present receptors. In one embodiment, one portion of the chimeracorresponds to, or is derived from the transmembrane domain of a T2Rpolypeptide of the invention. In another embodiment, one portion of thechimera corresponds to, or is derived from the one or more of thetransmembrane regions of a T2R polypeptide described herein, and theremaining portion or portions can come from another GPCR. Chimericreceptors are well known in the art, and the techniques for creatingthem and the selection and boundaries of domains or fragments of GProtein-Coupled Receptors for incorporation therein are also well known.Thus, this knowledge of those skilled in the art can readily be used tocreate such chimeric receptors. The use of such chimeric receptors canprovide, for example, a taste selectivity characteristic of one of thereceptors specifically disclosed herein, coupled with the signaltransduction characteristics of another receptor, such as a well knownreceptor used in prior art assay systems.

For example, a region such as a ligand-binding region, an extracellulardomain, a transmembrane domain, a transmembrane domain, a cytoplasmicdomain, an N-terminal domain, a C-terminal domain, or any combinationthereof, can be covalently linked to a heterologous protein. Forinstance, a T2R transmembrane region can be linked to a heterologousGPCR transmembrane domain, or a heterologous GPCR extracellular domaincan be linked to a T2R transmembrane region. Other heterologous proteinsof choice can include, e.g., green fluorescent protein,.beta.-galactosidase polypeptides, glutamate receptor, and the rhodopsinpolypeptides, e.g., N-terminal fragments of rhodopsin e.g., bovinerhodopsin.

It is also within the scope of the invention to use different host cellsfor expressing the T2Rs, fragments, or variants of the invention. Toobtain high levels of expression of a cloned gene or nucleic acid, suchas cDNAs encoding the T2Rs, fragments, or variants of the invention, oneof skill typically subclones the nucleic acid sequence of interest intoan expression vector that contains a strong promoter to directtranscription, a transcription/translation terminator, and if for anucleic acid encoding a protein, a ribosome binding site fortranslational initiation. Suitable bacterial promoters are well known inthe art and described, e.g., in Sambrook et al. Preferably, eukaryoticexpression systems are used to express the subject hT2R receptor.

Any of the well-known procedures for introducing foreign nucleotidesequences into host cells may be used. These include the use of calciumphosphate transfection, polybrene, protoplast fusion, electroporation,liposomes, microinjection, plasma vectors, viral vectors and any of theother well known methods for introducing cloned genomic DNA, cDNA,synthetic DNA or other foreign genetic material into a host cell (see,e.g., Sambrook et at) It is only necessary that the particular geneticengineering procedure used be capable of successfully introducing atleast one nucleic acid molecule into the host cell capable of expressingthe T2R, fragment, or variant of interest.

After the expression vector is introduced into the cells, thetransfected cells are cultured under conditions favoring expression ofthe receptor, fragment, or variant of interest, which is then recoveredfrom the culture using standard techniques. Examples of such techniquesare well known in the art. See, e.g., WO 00/06593, which is incorporatedby reference in a manner consistent with this disclosure.

Assays for Detection of Compounds that Modulate the Activity of a hT2RAccording to the Invention

Methods and compositions for determining whether a test compoundspecifically binds to a T2R polypeptide of the invention, both in vitroand in vivo are described below. Many aspects of cell physiology can bemonitored to assess the effect of ligand-binding to a naturallyoccurring or chimeric T2Rs. These assays may be performed on intactcells expressing a T2R polypeptide, on permeabilized cells, or onmembrane fractions produced by standard methods.

Taste receptors bind taste eliciting compounds and initiate thetransduction of chemical stimuli into electrical signals. An activatedor inhibited G protein will in turn alter the properties of targetenzymes, channels, and other effector proteins. Some examples are theactivation of cGMP phosphodiesterase by transducin in the visual system,adenylate cyclase by the stimulatory G protein, phospholipase C by Gqand other cognate G proteins, and modulation of diverse channels by Giand other G proteins. Downstream consequences can also be examined suchas generation of diacyl glycerol and IP3 by phospholipase C, and inturn, for calcium mobilization by IP3.

The subject hT2R proteins or polypeptides of the assay will typically beselected from a polypeptide having a sequence contained in the sequencelisting preceding the claims herein or fragments or conservativelymodified variants thereof.

Alternatively, the T2R proteins or polypeptides of the assay can bederived from a eukaryotic host cell, and can include an amino acidsequence having a certain percentage amino acid sequence identity tothese hT2R polypeptides or conservatively modified variants thereof.Generally, the amino acid sequence identity will be at least 30%preferably 30-40%, more specifically 50-60, 70%, 75%, 80%, 85%, 90%,95%, 96%, 97%, 98%, or 99%. Optionally, the T2R proteins or polypeptidesof the assays can comprise a region of a T2R polypeptide, such as anextracellular domain, transmembrane region, cytoplasmic domain,ligand-hinding domain, and the like. Optionally, as exemplified hereinthe T2R polypeptide, or a portion thereof; can be covalently linked to aheterologous protein to create a chimeric protein used in the assaysdescribed herein.

Modulators of T2R activity may be tested using T2R proteins orpolypeptides as described above, either recombinant or naturallyoccurring. The T2R proteins or polypeptides can be isolated, expressedin a cell, expressed in a membrane derived from a cell, expressed intissue or in an animal, either recombinant or naturally occurring. Forexample, tongue slices, dissociated cells from a tongue, transformedcells, or membranes can be used. Modulation can be tested using one ofthe in vitro or in vivo assays described herein.

Detection of Modulators

Compositions and methods for determining whether a test compoundspecifically binds to a T2R receptor of the invention, both in vitro andin vivo, are described below. Many aspects of cell physiology can bemonitored to assess the effect of ligand binding to a T2R polypeptide ofthe invention. These assays may be performed on intact cells expressinga chemosensory receptor, on permeabilized cells, or on membranefractions produced by standard methods or in vitro using de novosynthesized proteins.

In vivo, taste receptors bind to taste modulatory compounds and initiatethe transduction of chemical stimuli into electrical signals. Anactivated or inhibited G protein will in turn alter the properties oftarget enzymes, channels, and other effector proteins. Some examples arethe activation of cGMP phosphodiesterase by transducin in the visualsystem, adenylate cyclase by the stimulatory G protein, phospholipase Cby Gq and other cognate G proteins, and modulation of diverse channelsby Gi and other G proteins. Downstream consequences can also be examinedsuch as generation of diacyl glycerol and IP3 by phospholipase C, and inturn, for calcium mobilization by IP3.

Alternatively, the T2R proteins or polypeptides of the assay can bederived from a eukaryotic host cell and can include an amino acidsubsequence having amino acid sequence identity to the T2R polypeptidesdisclosed herein, or fragments or conservatively modified variantsthereof. Generally, the amino acid sequence identity will be at least 35to 50%, or optionally 75%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%.Optionally, the T2R proteins or polypeptides of the assays can comprisea domain of a T2R protein, such as an extracellular domain,transmembrane region, transmembrane domain, cytoplasmic domain,ligand-binding domain, and the like. Further, as described above, theT2R protein or a domain thereof can be covalently linked to aheterologous protein to create a chimeric protein used in the assaysdescribed herein.

Modulators of T2R receptor activity are tested using T2R proteins orpolypeptides as described above, either recombinant or naturallyoccurring. The T2R proteins or polypeptides can be isolated, expressedin a cell, expressed in a membrane derived from a cell, expressed intissue or in an animal, either recombinant or naturally occurring. Forexample, tongue slices, dissociated cells from a tongue, transformedcells, or membranes can be used. Modulation can be tested using one ofthe in vitro or in vivo assays described herein.

1. In Vitro Binding Assays

Taste transduction can also be examined in vitro with soluble or solidstate reactions, using the T2R polypeptides of the invention. In aparticular embodiment, T2R ligand-binding domains can be used in vitroin soluble or solid state reactions to assay for ligand binding.

It is possible that the ligand-binding domain may be formed by theN-terminal domain together with additional portions of the extracellulardomain, such as the extracellular loops of the transmembrane domain.

In vitro binding assays have been used with other GPCRs, such as themetabotropic glutamate receptors (see, e.g., Han and Hampson, J. Biol.Chem. 274:10008-10013 (1999)). These assays might involve displacing aradioactively or fluorescently labeled ligand, measuring changes inintrinsic fluorescence or changes in proteolytic susceptibility, etc.

Ligand binding to a T2R polypeptide according to the invention can betested in solution, in a bilayer membrane, optionally attached to asolid phase, in a lipid monolayer, or in vesicles. Binding of amodulator can be tested using, e.g., changes in spectroscopiccharacteristics (e.g., fluorescence, absorbance, refractive index)hydrodynamic (e.g., shape), chromatographic, or solubility properties.

In a preferred embodiment of the invention, a ^([35S])GTPγS bindingassay is used. As described above, upon activation of a GPCR, the Gαsubunit of the G protein complex is stimulated to exchange bound GDP forGTP. Ligand-mediated stimulation of G protein exchange activity can bemeasured in a biochemical assay measuring the binding of addedradioactively labeled ^([35S])GTPγS to the C protein in the presence ofa putative ligand. Typically, membranes containing the chemosensoryreceptor of interest are mixed with a G protein. Potential inhibitorsand/or activators and ^([35S])GTPγS are added to the assay, and bindingof^([35S])GTPγS to the G protein is measured. Binding can be measured byliquid scintillation counting or by any other means known in the art,including scintillation proximity assays (SPA). In other assays formats,fluorescently labeled GTPγS can be utilized.

2. Fluorescence Polarization Assays

In another embodiment, Fluorescence Polarization (“FP”) based assays maybe used to detect and monitor ligand binding. Fluorescence polarizationis a versatile laboratory technique for measuring equilibrium binding,nucleic acid hybridization, and enzymatic activity. Fluorescencepolarization assays are homogeneous in that they do not require aseparation step such as centrifugation, filtration, chromatography,precipitation, or electrophoresis. These assays are done in real time,directly in solution and do not require an immobilized phase.Polarization values can be measured repeatedly and after the addition ofreagents since measuring the polarization is rapid and does not destroythe sample. Generally, this technique can be used to measurepolarization values of fluorophores from low picomolar to micromolarlevels. This section describes how fluorescence polarization can be usedin a simple and quantitative way to measure the binding of ligands tothe T2R polypeptides of the invention.

When a fluorescently labeled molecule is excited with plane polarizedlight, it emits light that has a degree of polarization that isinversely proportional to its molecular rotation. Large fluorescentlylabeled molecules remain relatively stationary during the excited state(4 nanoseconds in the case of fluorescein) and the polarization of thelight remains relatively constant between excitation and emission. Smallfluorescently labeled molecules rotate rapidly during the excited stateand the polarization changes significantly between excitation andemission. Therefore, small molecules have low polarization values andlarge molecules have high polarization values. For example, asingle-stranded fluorescein-labeled oligonucleotide has a relatively lowpolarization value but when it is hybridized to a complementary strand,it has a higher polarization value. When using FP to detect and monitortaste eliciting compound-binding which may activate or inhibit thechemosensory receptors of the invention, fluorescence-labeled tasteeliciting compounds or auto-fluorescent taste eliciting compounds may beused.

Fluorescence polarization (P) is defined as:

$P = \frac{\lbrack {{Int}_{par} - {Int}_{{perp}\rbrack}} }{\lbrack {{Int}_{par} + {Int}_{perp}} \rbrack}$

Where Int_(par) is the intensity of the emission light parallel to theexcitation light plane and Int_(perp) is the intensity of the emissionlight perpendicular to the excitation light plane. P, being a ratio oflight intensities, is a dimensionless number. For example, the Beacon™and Beacon 2000™. System may be used in connection with these assays.Such systems typically express polarization in millipolarization units(1 Polarization Unit=1000 mP Units).

The relationship between molecular rotation and size is described by thePerrin equation and the reader is referred to Jolley, M. E. (1991) inJournal of Analytical Toxicology, pp. 236-240 incorporated by reference,which gives a thorough explanation of this equation. Summarily, thePerrin equation states that polarization is directly proportional to therotational relaxation time, the time that it takes a molecule to rotatethrough an angle of approximately 68.5°. Rotational relaxation time isrelated to viscosity (eta.), absolute temperature (T), molecular volume(V), and the gas constant (R) by the following equation:2(Rotational Relaxation Time)=3 V RT

The rotational relaxation time is small (˜nanosecond) for smallmolecules (e.g. fluorescein) and large (˜100 nanoseconds) for largemolecules (e.g. immunoglobulins). If viscosity and temperature are heldconstant, rotational relaxation time, and therefore polarization, isdirectly related to the molecular volume. Changes in molecular volumemay be due to interactions with other molecules, dissociation,polymerization, degradation, hybridization, or conformational changes ofthe fluorescently labeled molecule. For example, fluorescencepolarization has been used to measure enzymatic cleavage of largefluorescein labeled polymers by proteases, DNases, and RNases. It alsohas been used to measure equilibrium binding for protein/proteininteractions, antibody/antigen binding, and protein/DNA binding.

B. A. Solid State and Soluble High Throughput Assays

In yet another embodiment, the invention provides soluble assays using aT2R polypeptide; or a cell or tissue expressing a T2R polypeptide. Inanother embodiment, the invention provides solid phase based in vitroassays in a high throughput format, where the T2R polypeptide, or cellor tissue expressing the T2R polypeptide is attached to a solid phasesubstrate or a taste stimulating compound and contacted with a T2Rreceptor, and binding detected using an appropriate tag or antibodyraised against the T2R receptor.

In the high throughput assays of the invention, it is possible to screenup to several thousand different modulators or ligands in a single day.In particular, each well of a microtiter plate can be used to run aseparate assay against a selected potential modulator, or, ifconcentration or incubation time effects are to be observed, every 5-10wells can test a single modulator. Thus, a single standard microtiterplate can assay about 100 (e.g., 96) modulators. If 1536 well plates areused, then a single plate can easily assay from about 1000 to about 1500different compounds. It is also possible to assay multiple compounds ineach plate well. It is possible to assay several different plates perday; assay screens for up to about 6,000-20,000 different compounds ispossible using the integrated systems of the invention. More recently,microfluidic approaches to reagent manipulation have been developed.

The molecule of interest can be bound to the solid state component,directly or indirectly, via covalent or non-covalent linkage, e.g., viaa tag. The tag can be any of a variety of components. In general, amolecule which binds the tag (a tag binder) is fixed to a solid support,and the tagged molecule of interest (e.g., the taste transductionmolecule of interest) is attached to the solid support by interaction ofthe tag and the tag binder.

A number of tags and tag binders can be used, based upon known molecularinteractions well described in the literature. For example, where a taghas a natural hinder, for example, biotin, protein A, or protein G, itcan be used in conjunction with appropriate tag binders (avidin,streptavidin, neutravidin, the Fe region of an immunoglobulin, etc.).Antibodies to molecules with natural binders such as biotin are alsowidely available and appropriate tag binders (see, SIGMA Immunochemicals1998 catalogue SIGMA, St. Louis Mo.).

Similarly, any haptenic or antigenic compound can be used in combinationwith an appropriate antibody to form a tag/tag hinder pair. Thousands ofspecific antibodies are commercially available and many additionalantibodies are described in the literature. For example, in one commonconfiguration, the tag is a first antibody and the tag binder is asecond antibody which recognizes the first antibody. In addition toantibody-antigen interactions, receptor-ligand interactions are alsoappropriate as tag and tag-binder pairs. For example, agonists andantagonists of cell membrane receptors (e.g., cell receptor-ligandinteractions such as transferrin, c-kit, viral receptor ligands,cytokine receptors, chemokine receptors, interleukin receptors,immunoglobulin receptors and antibodies, the cadherin family, theintegrin family, the selectin family, and the like; see, e.g., Pigott &Power, The Adhesion Molecule Facts Book I (1993)). Similarly, toxins andvenoms, viral epitopes, hormones (e.g., opiates, steroids, etc.),intracellular receptors (e.g., which mediate the effects of varioussmall ligands, including steroids, thyroid hormone, retinoids andvitamin D; peptides), drugs, lectins, sugars, nucleic acids (both linearand cyclic polymer configurations), oligosaccharides, proteins,phospholipids and antibodies can all interact with various cellreceptors.

Synthetic polymers, such as polyurethanes, polyesters, polycarhonates,polyureas, polyamides, polyethyleneimines, polyarylene sulfides,polysiloxanes, polyimides, and polyacetates can also form an appropriatetag or tag binder. Many ether tag/tag binder pairs are also useful inassay systems described herein, as would be apparent to one of skillupon review of this disclosure.

Common linkers such as peptides, polyethers, and the like can also serveas tags, and include polypeptide sequences, such as poly gly sequencesof between about 5 and 200 amino acids. Such flexible linkers are knownto persons of skill in the art. For example, polyethylene glycol)linkers are available from Shearwater Polymers, Inc. Huntsville, Ala.These linkers optionally have amide linkages, sulfhydryl linkages, orheterofunctional linkages.

Tag binders are fixed to solid substrates using any of a variety ofmethods currently available. Solid substrates are commonly derivatizedor functionalized by exposing all or a portion of the substrate to achemical reagent which fixes a chemical group to the surface which isreactive with a portion of the tag binder. For example, groups which aresuitable for attachment to a longer chain portion would include amines,hydroxyl, thiol, and carboxyl groups. Aminoalkylsilanes andhydroxyalkylsilanes can be used to functionalize a variety of surfaces,such as glass surfaces. The construction of such solid phase biopolymerarrays is well described in the literature. See, e.g., Merrifield, J.Am. Chem. Soc., 85:2149-2154 (1963) (describing solid phase synthesisof, e.g., peptides); Geysen et al., J. Immun. Meth., 102:259-274 (1987)(describing synthesis of solid phase components on pins); Frank &Doring, Tetrahedron, 44:60316040 (1988) (describing synthesis of variouspeptide sequences on cellulose disks); Fodor et al., Science,251:767-777 (1991); Sheldon et al., Clinical Chemistry, 39(4):718-719(1993); and Kozal et al., Nature Medicine, 2(7):753759 (1996) (alldescribing arrays of biopolymers fixed to solid substrates).Non-chemical approaches for fixing tag binders to substrates includeother common methods, such as heat, cross-linking by UV radiation, andthe like.

3. Cell-based Assays

In one preferred embodiment, a T2R protein is expressed in a eukaryoticcell either in unmodified forms or as chimeric, variant or truncatedreceptors with or preferably without a heterologous, chaperone sequencethat facilitates its maturation and targeting through the secretorypathway. Such T2R polypeptides can be expressed in any eukaryotic cell,such as HEK-293 cells. Preferably, the cells comprise a functional Gprotein, e.g., G._(α15), or a chimeric G._(α16), gustducin or transducinor a chimeric G protein such as G16gust44 that is capable of couplingthe chimeric receptor to an intracellular signaling pathway or to asignaling protein such as phospholipase C. Activation of T2R receptorsin such cells can be detected using any standard method, such as bydetecting changes in intracellular calcium by detecting FURA-2 dependentfluorescence in the cell. Such an assay is the basis of the experimentalfindings presented in this application.

Activated GPCR receptors often are substrates for kinases thatphosphorylate the C-terminal tail of the receptor (and possibly othersites as well). Thus, activators will promote the transfer of ³²P fromradiolabeled ATP to the receptor, which can be assayed with ascintillation counter. The phosphorylation of the C-terminal tail willpromote the binding of arrestin-like like proteins and will interferewith the binding of G proteins. For a general review of GPCR signaltransduction and methods of assaying signal transduction, see, e.g.,Methods in Enzymology, vols. 237 and 238 (1994) and volume 96 (1983);Bourne et al., Nature, 10:349:117-27 (1991); Bourne et al., Nature,348:125-32 (1990); Pitcher et al., Annu. Rev. Biochem., 67:653-92(1998).

T2R modulation may be assayed by comparing the response of T2Rpolypeptides treated with a putative T2R modulator to the response of anuntreated control sample or a sample containing a known “positive”control. Such putative T2R modulators can include molecules that eitherinhibit or activate T2R polypeptide activity. In one embodiment, controlsamples treated with a compound that activates the T2R are assigned arelative T2R activity value of 100. Inhibition of a T2R polypeptide isachieved when the T2R activity value relative to the control sample isabout 90%, optionally 50%, optionally 25-0%. Activation of a T2Rpolypeptide is achieved when the T2R activity value relative to thecontrol is 110%, optionally 150%, 200-500%, or 1000-2000%.

Changes in ion flux may be assessed by determining changes in ionicpolarization (i.e., electrical potential) of the cell or membraneexpressing a T2R polypeptide. One means to determine changes in cellularpolarization is by measuring changes in current (thereby measuringchanges in polarization) with voltage-clamp and patch-clamp techniques(see, e.g., the “cell-attached” mode, the “inside-out” mode, and the“whole cell” mode, e.g., Ackerman et al., New Engl. J Med.,336:1575-1595 (1997)). Whole cell currents are conveniently determinedusing the standard. Other known assays include: radiolabeled ion fluxassays and fluorescence assays using voltage-sensitive dyes (see, e.g.,Vestergarrd-Bogind et al., J. Membrane Biol., 88:67-75 (1988); Gonzales& Tsien, Chem. Biol, 4:269-277 (1997); Daniel et al., J. Pharmacoi.Meth., 25:185-193 (1991); Holevinsky et al., J. Membrane Biology,137:59-70 (1994)).

The effects of the test compounds upon the function of the polypeptidescan be measured by examining any of the parameters described above. Anysuitable physiological change that affects GPCR activity can be used toassess the influence of a test compound on the polypeptides of thisinvention. When the functional consequences are determined using intactcells or animals, one can also measure a variety of effects such astransmitter release, hormone release, transcriptional changes to bothknown and uncharacterized genetic markers (e.g., northern blots),changes in cell metabolism such as cell growth or pH changes, andchanges in intracellular second messengers such as Ca2+, IP3, cGMP, orcAMP.

Preferred assays for GPCRs include cells that are loaded with ion orvoltage sensitive dyes to report receptor activity. Assays fordetermining activity of such receptors can also use known agonists andantagonists for other G protein-coupled receptors as controls to assessactivity of tested compounds. In assays for identifying modulatorycompounds (e.g., agonists, antagonists), changes in the level of ions inthe cytoplasm or membrane voltage will be monitored using an ionsensitive or membrane voltage fluorescent indicator, respectively. Amongthe ion-sensitive indicators and voltage probes that may be employed arethose disclosed in the Molecular Probes 1997 Catalog. For Gprotein-coupled receptors, promiscuous G proteins such as G_(α15) andG_(α16) can be used in the assay of choice (Wilkie et al., Proc. Nat'lAcad. Sci., 88:10049-10053 (1991)). Alternatively, other G proteins suchas gustducin, transducin and chimeric G proteins such as Gα6gust44 orGalphal16t25 may be used.

Receptor activation initiates subsequent intracellular events, e.g.,increases in second messengers. Activation of some G protein-coupledreceptors stimulates the formation of inositol triphosphate (IP3)through phospholipase C-mediated hydrolysis of phosphatidylinositol(Berridge & Irvine, Nature, 312:315-21 (1984)). IP3 in turn stimulatesthe release of intracellular calcium ion stores. Thus, a change incytoplasmic calcium ion levels, or a change in second messenger levelssuch as IP3 can be used to assess G protein-coupled receptor function.Cells expressing such G protein-coupled receptors may exhibit increasedcytoplasmic calcium levels as a result of contribution from both calciumrelease from intracellular stores and extracellular calcium entry viaplasma membrane ion channels,

In a preferred embodiment, T2R polypeptide activity is measured byexpressing T2R gene in a heterologous cell with a promiscuous G proteinthat links the receptor to a phospholipase C signal transduction pathway(see Offermanns & Simon, J. Biol. Chem., 270:15175-15180 (1995)).Preferably, the cell line is HEK-293 (which does not normally expressT2R genes) and the promiscuous G protein is G_(α15) (Offermanns & Simon,supra) or a chimeric G protein such as Gα16gust44. Modulation of tastetransduction is assayed by measuring changes in intracellular Ca²⁻levels, which change in response to modulation of the T2R signaltransduction pathway via administration of a molecule that associateswith the T2R polypeptide. Changes in Ca²⁺ levels are optionally measuredusing fluorescent Ca²⁺ indicator dyes and fluorimetric imaging.

In another embodiment, phosphatidyl inositol (PI) hydrolysis can beanalyzed according to U.S. Pat. No. 5,436,128, herein incorporated byreference. Briefly, the assay involves labeling of cells with3H-myoinositol for 48 or more hrs. The labeled cells are treated with atest compound for one hour. The treated cells are lysed and extracted inchloroform-methanol-water after which the inositol phosphates wereseparated by ion exchange chromatography and quantified by scintillationcounting. Fold stimulation is determined by calculating the ratio of cpmin the presence of agonist, to cpm in the presence of buffer control.Likewise, fold inhibition is determined by calculating the ratio of cpmin the presence of antagonist, to cpm in the presence of buffer control(which may or may not contain an agonist).

Other receptor assays can involve determining the level of intracellularcyclic nucleotides, e.g., cAMP or cGMP. In cases where activation of thereceptor results in a decrease in cyclic nucleotide levels, it may bepreferable to expose the cells to agents that increase intracellularcyclic nucleotide levels, e.g., forskolin, prior to adding areceptor-activating compound to the cells in the assay. In oneembodiment, the changes in intracellular cAMP or cGMP can be measuredusing immunoassays. The method described in Offermanns & Simon, J. Bio.Chem., 270:15175-15180 (1995), may be used to determine the level ofcAMP. Also, the method described in Felley-Bosco et al., Am. J. Resp.Cell and Mol. Biol., 11:159-164 (1994), may be used to determine thelevel of cGMP. Further, an assay kit for measuring cAMP and/or cGMP isdescribed in U.S. Pat. No. 4,115,538, herein incorporated by reference.

In another embodiment, transcription levels can be measured to assessthe effects of a test compound on signal transduction. A host cellcontaining T2R polypeptide of interest is contacted with a test compoundfor a sufficient time to effect any interactions, and then the level ofgene expression is measured. The amount of time to effect suchinteractions may be empirically determined, such as by running a timecourse and measuring the level of transcription as a function of time.The amount of transcription may be measured by using any method known tothose of skill in the art to be suitable. For example, mRNA expressionof the protein of interest may be detected using northern blots or theirpolypeptide products may be identified using immunoassays.Alternatively, transcription based assays using a reporter gene may heused as described in U.S. Pat. No. 5,436,128, herein incorporated byreference. The reporter genes can be, e.g., chloramphenicolacetyltransferase, luciferase, beta-galactosidase, beta-lactamase andalkaline phosphatase. Furthermore, the protein of interest can be usedas an indirect reporter via attachment to a second reporter such asgreen fluorescent protein (see, e.g., Mistili & Spector, NatureBiotechnology, 15:961-964 (1997)).

The amount of transcription is then compared to the amount oftranscription in either the same cell in the absence of the testcompound, or it may be compared with the amount of transcription in asubstantially identical cell that lacks the T2R polypeptide(s) ofinterest. A substantially identical cell may be derived from the samecells from which the recombinant cell was prepared but which had notbeen modified by introduction of heterologous DNA. Any difference in theamount of transcription indicates that the test compound has in somemanner altered the activity of the T2R polypeptide of interest.

4. Transgenic Non-human Animals Expressing Chemosensory Receptors

Non-human animals expressing one or more taste receptor sequences of theinvention can also be used for receptor assays. Such expression can beused to determine whether a test compound specifically binds to amammalian taste transmembrane receptor complex in vivo by contacting anon-human animal stably or transiently transfected with nucleic acidsencoding chemosensory receptors or ligand-binding regions thereof with atest compound and determining whether the animal reacts to the testcompound by specifically binding to the receptor polypeptide complex.

Animals transfected or infected with the vectors of the invention areparticularly useful for assays to identify and characterize tastestimuli that can bind to a specific or sets of receptors. Suchvector-infected animals expressing human taste receptor sequences can beused for in vivo screening of taste stimuli and their effect on, e.g.,cell physiology (e.g., on taste neurons), on the CNS, or behavior.

Means to infect/express the nucleic acids and vectors, eitherindividually or as libraries, are well known in the art. A variety ofindividual cell, organ, or whole animal parameters can be measured by avariety of means. The T2R sequences of the invention can be for exampleexpressed in animal taste tissues by delivery with an infecting agent,e.g., adenovirus expression vector.

The endogenous taste receptor genes can remain functional and wild-type(native) activity can still be present. In other situations, where it isdesirable that all taste receptor activity is by the introducedexogenous hybrid receptor, use of a knockout line is preferred. Methodsfor the construction of non-human transgenic animals, particularlytransgenic mice, and the selection and preparation of recombinantconstructs for generating transformed cells are well known in the art.

Construction of a “knockout” cell and animal is based on the premisethat the level of expression of a particular gene in a mammalian cellcan be decreased or completely abrogated by introducing into the genuinea new DNA sequence that serves to interrupt some portion of the DNAsequence of the gene to be suppressed. Also, “gene trap insertion” canbe used to disrupt a host gene, and mouse embryonic stem (ES) cells canbe used to produce knockout transgenic animals (see, e.g., Holzschu,Transgenic Res 6:97-106 (1997)). The insertion of the exogenous istypically by homologous recombination between complementary nucleic acidsequences. The exogenous sequence is some portion of the target gene tobe modified, such as exonic, intronic or transcriptional regulatorysequences, or any genomic sequence which is able to affect the level ofthe target gene's expression; or a combination thereof. Gene targetingvia homologous recombination in pluripotential embryonic stem cellsallows one to modify precisely the genomic sequence of interest. Anytechnique can be used to create, screen for, propagate, a knockoutanimal, e.g., see Bijvoet, Hum. Mol. Genet. 7:53-62 (1998); Moreadith,J. Mol. Med. 75:208-216 (1997); Tojo, Cytotechnology 19:161-165 (1995);Mudgett, Methods Mol. Biol. 48:167-184 (1995); Longo, Transgenic Res.6:321-328 (1997); U.S. Pat. Nos. 5,616,491; 5,464,764; 5,631,153;5,487,992; 5,627,059; 5,272,071; WO 91/09955; WO 93/09222; WO 96129411;WO 95/81560; WO 91/12650.

The nucleic acids of the invention can also be used as reagents toproduce “knockout” human cells and their progeny. Likewise, the nucleicacids of the invention can also be used as reagents to produce“knock-ins” in mice. The human or rat T2R gene sequences can replace theorthologs T2R in the mouse genome. In this way, a mouse expressing ahuman or rat T2R is produced. This mouse can then be used to analyze thefunction of human or rat T2Rs, and to identify ligands for such T2Rs.

Modulators

The compounds tested as modulators of a T2R family member can be anysmall chemical compound, or a biological entity, such as a protein,sugar, nucleic acid or lipid. Alternatively, modulators can begenetically altered versions of a T2R family member. Typically, testcompounds may be small chemical molecules and peptides. Essentially anychemical compound can be used as a potential modulator or ligand in theassays of the invention, although most often compounds can be dissolvedin aqueous or organic (especially DMSO-based) solutions are used. Theassays may be designed to screen large chemical libraries by automatingthe assay steps and providing compounds from any convenient source toassays, which are typically run in parallel (e.g., in microtiter formatson microtiter plates in robotic assays). It will be appreciated thatthere are many suppliers of chemical compounds, including Sigma (St.Louis, Mo.), Aldrich (St. Louis, Mo.), Sigma-Aldrich (St. Louis, Mo.),Fluka Chemika-Biochemica Analytika (Buchs, Switzerland) and the like.

In one embodiment, high throughput screening methods involve providing acombinatorial chemical or peptide library containing a large number ofpotential therapeutic compounds (potential modulator or ligandcompounds). Such “combinatorial chemical libraries” or “ligandlibraries” are then screened in one or more assays, as described herein,to identify those library members (particular chemical species orsubclasses) that display a desired characteristic activity. Thecompounds thus identified can serve as conventional “lead compounds” orcan themselves be used as potential or actual consumer products.

A combinatorial chemical library is a collection of diverse chemicalcompounds generated by either chemical synthesis or biologicalsynthesis, by combining a number of chemical “building blocks” such asreagents. For example, a linear combinatorial chemical library such as apolypeptide library is formed by combining a set of chemical buildingblocks (amino acids) in every possible way for a given compound length(i.e., the number of amino acids in a polypeptide compound). Millions ofchemical compounds can be synthesized through such combinatorial mixingof chemical building blocks.

Preparation and screening of combinatorial chemical libraries is wellknown to those of skill in the art. Such combinatorial chemicallibraries include, but are not limited to, peptide libraries (see, e.g.,U.S. Pat. No. 5,010,175, Furka, Int. J. Pept. Prot. Res., 37:487-93(1991) and Houghton et al., Nature, 354:84-88 (1991)). Other chemistriesfor generating chemical diversity libraries can also be used. Suchchemistries include, but are not limited to: peptoids (e.g., WO91/19735), encoded peptides (e.g., WO 93/20242), random bio-oligomers(e.g., WO 92/00091), benzodiazepines (e.g., U.S. Pat. No. 5,288,514),diversomers such as hydantoins, benzodiazepines and dipeptides (Hobbs etal., PNAS., 90:6909-13 (1993)), vinylogous polypeptides (Hagihara etal., J. Amer. Chem. Soc., 114:6568 (1992)), nonpeptidal peptidomimeticswith glucose scaffolding (Hirschmann et al., J. Amer. Chem. Soc.,114:9217-18 (1992)), analogous organic syntheses of small compoundlibraries (Chen et al., J. Amer. Chem. Soc., 116:2661 (1994)),oligocarbamates (Cho et al., Science, 261:1303 (1993)), peptidylphosphonates (Campbell et al., J. Org. Chem., 59:658 (1994)), nucleicacid libraries (Ausubel, Berger, and Sambrook, all supra), peptidenucleic acid libraries (U.S. Pat. No. 5,539,083), antibody libraries(Vaughn et al., Nature Biotechnology, 14(3):309-14 (1996) andPCT/US96/10287), carbohydrate libraries (Liang et al., Science,274:1520-22 (1996) and U.S. Pat. No. 5,593,853), small organic moleculelibraries (benzodiazepines, Baum, C&EN, January 18, page 33 (1993);thiazolidinones and metathiazanones, U.S. Pat. No. 5,549,974;pynrolidines, U.S. Pat. Nos. 5,525,735 and 5,519,134; morpholinocompounds, U.S. Pat. No. 5,506,337; benzodiazepines, U.S. Pat. No.5,288,514, and the like).

Devices for the preparation of combinatorial libraries are commerciallyavailable (see, e.g., 357 MPS, 390 MPS (Advanced Chem Tech, LouisvilleKy.), Symphony (Rainin, Woburn, Mass.), 433A (Applied Biosystems, FosterCity, Calif.), 9050 Plus (Millipore, Bedford, Mass.)). In addition,numerous combinatorial libraries are themselves commercially available(see, e.g., ComGenex, Princeton, N.J.; Tripos, Inc., St. Louis, Mo.; 3DPharmaceuticals, Exton, Pa.; Martek Biosciences; Columbia, Md.; etc.).

In one aspect of the invention, the T2R modulators can be used in anyfood product, confectionery, pharmaceutical composition, or ingredientthereof to thereby modulate the taste of the product, composition, oringredient in a desired manner. For instance, T2R modulators thatenhance bitter taste sensation can be added to provide a bitter taste toa product or composition, while T2R modulators which block bitter tastesensations can be added to block the bitter taste of a product orcomposition. Also, the invention provides means of identifying bittercompounds found in foods, beverages and medicinals and producing tasteimproved foods, beverages and medicinals lacking or having a reducedquantity thereof.

Use of Compounds Identified by the Invention

Compounds identified according to the invention may be added to foods,beverages, cosmetics or medicinal compositions to modulate, preferablyblock bitter taste triggered by activation at least one of one of hT2R1,hT2R3, hT2R4, hT2R5, hT2R7, hT2R8, hT2R9A, hT2R9V, hT2R10, hT2R13,hT2R14, hT2R16, hT2R44, hT2R54, hT2R55, hT2R61, hT2R64, hT2R65, hT2R67,hT2R71, hT2R75 and hT2R76, and hT2R54 by at least one of the bittercompounds contained in FIG. 2 or structurally related compounds or otherbitter compounds, e.g., compounds found in foods and beverages ormedicinals or cosmetics that elicit a bitter taste perception.

As noted previously, preferably, the taste modulatory properties,preferably bitter taste blocking properties of compounds identified inthe subject T2R cell-based assays will be confirmed in human or animaltaste tests, preferably human taste tests.

Kits

T2R genes and their homologs are useful tools for identifying tastereceptor cells, for forensics and paternity determinations, and forexamining taste transduction. T2R family member-specific reagents thatspecifically hybridize to T2R nucleic acids, such as T2R probes andprimers, and T2R specific reagents that specifically bind to a T2Rprotein, e.g., T2R antibodies are used to examine taste cell expressionand taste transduction regulation.

Nucleic acid assays for the presence of DNA and RNA for a T2R familymember in a sample include numerous techniques are known to thoseskilled in the art, such as southern analysis, northern analysis, dotblots, RNase protection, S1 analysis, amplification techniques such asPCR, and in situ hybridization. In in situ hybridization, for example,the target nucleic acid is liberated from its cellular surroundings insuch as to be available for hybridization within the cell whilepreserving the cellular morphology for subsequent interpretation andanalysis. The following articles provide an overview of the art of insitu hybridization: Singer et al., Biotechniqucs, 4:230250 (1986); Haaseet al., Methods in Virology, vol. VII, 189-226 (1984); and Names et al.,eds., Nucleic Acid Hybridization: A Practical Approach (1987). Inaddition, a T2R protein can be detected with the various immunoassaytechniques described above. The test sample is typically compared toboth a positive control (e.g., a sample expressing a recombinant T2Rprotein) and a negative control.

The present invention also provides for kits for screening formodulators of T2R family members. Such kits can be prepared from readilyavailable materials and reagents. For example, such kits can compriseany one or more of the following materials: T2R nucleic acids orproteins, reaction tubes, and instructions for testing T2R activity.Optionally, the kit contains a functional T2R polypeptide. A widevariety of kits and components can be prepared according to the presentinvention, depending upon the intended user of the kit and theparticular needs of the user.

Having now generally described the invention, the same will be morereadily understood by reference to the following examples, which areprovided by way of illustration and are not intended as limiting. It isunderstood that various modifications and changes can be made to theherein disclosed exemplary embodiments without departing from the spiritand scope of the invention.

EXAMPLES Example 1

In this example, HEK-293 cells were produced that stably or transientlyexpress one of hT2R1, hT2R3, hT2R4, hT2R5, hT2R7, hT2R8, hT2R9, hT2R10,hT2R13, hT2R14, hT2R16, hT2R44, hT2R50, hT2R51, hT2R54, hT2R55, hT2R61,hT2R64, hT2R65, hT2R67, hT2R71, hT2R75 and hT2R76 and a chimeric Gprotein that couples therewith (sequences for these hT2Rs and chimeric Gprotein are contained in the Sequence Listing preceding the claims) andthese cells were screened against different bitter ligands to identifycompounds that specifically activate each of these bitter tastereceptors.

More specifically, activation of these receptors by different bitterligands is measured in a cell-based assay detecting changes inintracellular calcium concentration. In brief, human embryonic kidneycells are seeded into 48-well tissue culture plates. 24 hours laterthese seeded HEK-293 cells are transiently transfected with anexpression plasmid (pEAK10) containing a sequence encoding a chimeric Gprotein (G16gust44) (SEQ ID NO:2) and are also arc transientlytransfected with a pEAK10 expression plasmid containing a particularhT2R nucleic acid sequence either by Ca²⁺ phosphate or lipid-basedsystems. Additionally, the hT2R sequences contained in the pEAK10plasmid are each engineered to contain a Rho-35 tag or a SSTR-3 tagupstream of the hT2R sequence and therefor express an N-terminal tag of35 amino acids of the rhodopsin protein (SEQ ID NO:1) or of 45 aminoacids of SSTR-3 protein,

After another 24 hours the transiently transfected cells are incubatedwith a fluorescent dye specific for calcium (Fluo-4; Molecular Probes).These loaded cells are then exposed to different bitter molecules.Activation of the hT2R by a bitter ligand results in the activation ofG16gust44, which leads to calcium mobilization within the cells.Thereupon this change in intracellular calcium causes the fluorescentdye to emit a detectable change in the fluorescence which is monitoredusing fluorescence microscopy. Particularly, this increase in calciumconcentration changes the fluorescence properties of the calcium dyeinside the cells and these changes are monitored using fluorescencemicroscopy and a specifically designed software (Imaging Workbench,Axon).

Also, the assay can be performed with the automated fluorimetric imagingsystem (FLIPR) using a slightly different protocol. In this protocol anHEK293 cell line stably expressing G16gust44 was transfected with anhT2R expression plasmid and 24 hours later the cells are dye loaded andanalyzed on FLIPR.

Additionally, once a ligand is identified for a particular hT2R usingthis approach, a HEK-293 cell line is generated that stably expressesthe particular hT2R and the chimeric G protein G16gust44 thereby

A large number of bitter ligands were assayed against theabove-identified hT2Rs at the concentrations set forth therein. Theresults of these assays for each of the hT2Rs are summarized in FIG. 2.It can be seen that each of the tested hT2Rs was found to bespecifically activated by at least one bitter ligand. As can be seenfrom the tabulated results, typically, a particular hT2R responds to anumber of different bitter ligands having divergent structures. Also theresults in FIG. 2 show that many of the bitter ligands specificallyactivate more than one hT2R. The concentrations used for these bitterligands are contained in FIG. 3.

Example 2 hT2R9 Allelic Variants and Assays

The deorphaning of hT2R9 and of other human T2R is reported in theprevious example. This example relates specifically to the hT2R9sequences contained in Sequence in the Sequence Listing that precedesthe claims herein. With respect thereto it has been reported by Kim etal (Hum Mutat. 2005 September; 26:199-204) that there are 2 haplotypesof hT2R9. The protein and DNA sequences of both T2R9 haplotypes (hT2R9Aand hT2R9V) are contained in SEQ ID NO: 13-16 in the Sequence Listing.The one we refer to as hT2R9V, appears to be a non-functional allelewith the bitter ligands tested, while the other allele, hT2R9A, isfunctional with tested bitter ligands. The 2 alleles differ by one aminoacid residue at amino acid position 187, while hT2R9A contains an Alaresidue, hT2R9V instead contains a Val residue. According to Kim et al(Hum Mutat. 2005 September; 26:199-204), those 2 haplotypes representabout 91% of the human population (hT2R9V 55.5%, and hT2R9A 35.5%).

As described in the foregoing example 23 different human taste receptorswere successfully deorphaned including hT2R9. The 2 major haplotypes ofhT2R9 are referred to here as hT2R9V and hT2R9A. The nucleic and aminoacid sequences corresponding thereto are contained in the SequenceListing that precedes the claims.

The hT2R9V sequence was initially cloned. Thereafter, hT2R9A sequencewas cloned and both were expressed using a Rho-tagged expression vectorand the same HEK-293 cell-based assays for identifying T2R9 ligands.Both of these hT2R9 sequences were evaluated in these assays using thesame ligands. It was observed that hT2R9A demonstrated activity whencontacted with ofloxacin whereas hT2R9V or mock-transfected HEK-293cells did not under the same assay conditions (See FIG. 4).

In addition the same T2R9 sequences were assayed using a differentexpression construct containing a SSTR3 tag. (Bufe et al., Nat. Genet.32:397-400 (2002)). These results generated the dose-dependent curvescontained in FIG. 5 using transiently transfected cells and FLIPR. Theseresults were in accord with the previous experiment and suggest thathT2R9A is functional, i.e., it responds to specific bitter ligandswhereas hT2R9V does not respond to the same bitter ligands. As notedabove, T2Rs and T1Rs are both known to be expressed in gastrointestinalcells and are believed to potentially play a role in digestive andmetabolic functions and diseases. Therefore, either or both of theseT2R9 alleles may be useful in therapeutic screening assays. Also, suchallelic variation in hT2R sequences including hT2R9 may be involved inthe disparate responses to bitter ligands in some individuals, e.g.,between the group of people carrying hT2R9A versus the group who arehomozygous for the hT2R9V allele.

The invention claimed is:
 1. An assay for identifying a compound whichmodulates a human T2R8 bitter taste receptor which comprises a bittertaste receptor polypeptide possessing at least 90% sequence identity tothe polypeptide of SEQ ID NO:12, wherein said bitter taste receptorpolypeptide specifically responds to sucrose octaacetate, which assaycomprises: i) screening a compound for its effect on at least one bittercompound to induce the activation of a bitter taste receptor polypeptidepossessing at least 90% sequence identity to the polypeptide of SEQ IDNO:12, wherein such bitter taste receptor polypeptide is activated bysucrose octaacetate, ii) determining whether said compound modulates theactivation of said bitter taste receptor by sucrose octaacetate; andiii) identifying said compound as a potential bitter taste modulator ifit modulates the activation of said human bitter taste receptor bysucrose octaacetate.
 2. The assay of claim 1, wherein step (ii) detectsmodulation of the activity of the bitter taste receptor polypeptide bydetecting at least one of the following: membrane potential andconductance changes; ion flux changes; changes in intracellular secondmessengers; changes in intracellular ion levels; or neurotransmitterrelease.
 3. The assay of claim 2, wherein modulation of the activity ofthe bitter taste receptor polypeptide is determined by detecting changesin intracellular cation concentrations.
 4. The assay of claim 2, whereinmodulation of the activity of the bitter taste receptor polypeptide isdetermined by detecting changes in intracellular second messengersselected from any of cAMP, cGMP, and inositol triphosphate (IP3).
 5. Theassay of claim 1, wherein the bitter taste receptor polypeptidepossesses at least 95% sequence identity to a polypeptide comprising thesequence of SEQ ID NO:12.
 6. The assay of claim 1, wherein the bittertaste receptor polypeptide possesses at least 96% sequence identity tothe polypeptide of SEQ ID NO:12.
 7. The assay of claim 1, wherein thebitter taste receptor polypeptide possesses at least 97% sequenceidentity to the polypeptide of SEQ ID NO:12.
 8. The assay of claim 1,wherein the bitter taste receptor polypeptide possesses at least 98%sequence identity to the polypeptide of SEQ ID NO:12.
 9. The assay ofclaim 1, wherein the bitter taste receptor polypeptide possesses atleast 99% sequence identity to a polypeptide to the polypeptide of SEQID NO:12.
 10. The assay of claim 1, wherein the bitter taste receptorpolypeptide possesses 100% sequence identity to the polypeptide of SEQID NO:12.